diff --git a/CommandLines.cpp b/CommandLines.cpp
index c9e6eda..5e519f5 100644
--- a/CommandLines.cpp
+++ b/CommandLines.cpp
@@ -49,6 +49,8 @@ static ko_longopt_t long_options[] = {
     { "kpt-rate",     ko_required_argument, 335},
     { "ul-rate",     ko_required_argument, 336},
     { "dbg-het-cnt",     ko_no_argument, 337},
+    { "ul-tip",     ko_required_argument, 338},
+    { "low-het",     ko_no_argument, 339},
 	{ 0, 0, 0 }
 };
 
@@ -146,8 +148,10 @@ void Print_H(hifiasm_opt_t* asm_opt)
     fprintf(stderr, "  Ultra-Long-integration (beta):\n");
     fprintf(stderr, "    --ul FILEs   file names of Ultra-Long reads [r1.fq,r2.fq,...]\n");
     fprintf(stderr, "    --ul-rate    FLOAT\n");
-    fprintf(stderr, "                 similarity threshold for UL-to-HiFi alignment [%.3g]\n", asm_opt->ul_error_rate);
-    
+    fprintf(stderr, "                 error rate of Ultra-Long reads [%.3g]\n", asm_opt->ul_error_rate);
+    fprintf(stderr, "    --ul-tip     INT\n");
+    fprintf(stderr, "                 remove tip unitigs composed of <=INT reads for the UL assembly [%d]\n", asm_opt->max_short_ul_tip);
+    // fprintf(stderr, "    --low-het    enable it for genomes with very low het heterozygosity rate (<0.0001%%)\n");
 
     fprintf(stderr, "Example: ./hifiasm -o NA12878.asm -t 32 NA12878.fq.gz\n");
     fprintf(stderr, "See `https://hifiasm.readthedocs.io/en/latest/' or `man ./hifiasm.1' for complete documentation.\n");
@@ -204,6 +208,7 @@ void init_opt(hifiasm_opt_t* asm_opt)
     asm_opt->min_overlap_Len = 50;
     asm_opt->min_overlap_coverage = 0;
     asm_opt->max_short_tip = 3;
+    asm_opt->max_short_ul_tip = 6;
     asm_opt->min_cnt = 2;
     asm_opt->mid_cnt = 5;
     asm_opt->purge_level_primary = 3;
@@ -246,6 +251,7 @@ void init_opt(hifiasm_opt_t* asm_opt)
     asm_opt->ul_error_rate_hpc = 0.2;
     asm_opt->ul_ec_round = 3;
     asm_opt->is_dbg_het_cnt = 0;
+    asm_opt->is_low_het_ul = 0;
 }
 
 void destory_enzyme(enzyme* f)
@@ -763,6 +769,8 @@ int CommandLine_process(int argc, char *argv[], hifiasm_opt_t* asm_opt)
         else if (c == 335) asm_opt->kpt_rate = atof(opt.arg);
         else if (c == 336) asm_opt->ul_error_rate = atof(opt.arg);
         else if (c == 337) asm_opt->is_dbg_het_cnt = 1;
+        else if (c == 338) asm_opt->max_short_ul_tip = atol(opt.arg);
+        else if (c == 339) asm_opt->is_low_het_ul = 1;
         else if (c == 'l')
         {   ///0: disable purge_dup; 1: purge containment; 2: purge overlap
             asm_opt->purge_level_primary = asm_opt->purge_level_trio = atoi(opt.arg);
diff --git a/CommandLines.h b/CommandLines.h
index eb062e8..013c611 100644
--- a/CommandLines.h
+++ b/CommandLines.h
@@ -4,7 +4,7 @@
 #include <pthread.h>
 #include <stdint.h>
 
-#define HA_VERSION "0.18.1-r466"
+#define HA_VERSION "0.18.2-r467"
 
 #define VERBOSE 0
 
@@ -79,6 +79,7 @@ typedef struct {
     int min_overlap_Len;
     int min_overlap_coverage;
     int max_short_tip;
+    int max_short_ul_tip;
     int min_cnt;
     int mid_cnt;
     int purge_level_primary;
@@ -128,6 +129,7 @@ typedef struct {
     double ul_error_rate, ul_error_rate_low, ul_error_rate_hpc;
     int32_t ul_ec_round;
     uint8_t is_dbg_het_cnt;
+    uint8_t is_low_het_ul;
 } hifiasm_opt_t;
 
 extern hifiasm_opt_t asm_opt;
diff --git a/Overlaps.cpp b/Overlaps.cpp
index f86158a..95c7eb6 100644
--- a/Overlaps.cpp
+++ b/Overlaps.cpp
@@ -3594,7 +3594,7 @@ uint32_t startNode, uint32_t endNode, int max_dist, buf_t* bub)
                    {
                        Ns_first[0] = av[0].v; Ns_first[1] = av[1].v;
                        f1 = asg_bub_end_finder_with_del_advance(g, Ns_first, 2, max_dist, 
-                       bub, 0, (u_int32_t)-1, &convex1);
+                       bub, 0, (uint32_t)-1, &convex1);
                        l1 = min_thres + 10;
                    }
 
@@ -3602,7 +3602,7 @@ uint32_t startNode, uint32_t endNode, int max_dist, buf_t* bub)
                    {
                        Ns_second[0] = aw[0].v; Ns_second[1] = aw[1].v;
                        f2 = asg_bub_end_finder_with_del_advance(g, Ns_second, 2, max_dist, 
-                       bub, 0, (u_int32_t)-1, &convex2);
+                       bub, 0, (uint32_t)-1, &convex2);
                        l2 = min_thres + 10;
                    }
 
@@ -4034,7 +4034,7 @@ uint32_t startNode, uint32_t endNode, int max_dist, buf_t* bub)
             {
                 Ns_first[0] = Nodes1[0]^1; Ns_first[1] = Nodes2[1]^1;
                 f1 = asg_bub_end_finder_with_del_advance(g, Ns_first, 2, max_dist, 
-                bub, 0, (u_int32_t)-1, &convex1);
+                bub, 0, (uint32_t)-1, &convex1);
                 l1 = min_thres + 10;
             }
 
@@ -4042,7 +4042,7 @@ uint32_t startNode, uint32_t endNode, int max_dist, buf_t* bub)
             {
                 Ns_second[0] = Nodes2[0]^1; Ns_second[1] = Nodes1[1]^1;
                 f2 = asg_bub_end_finder_with_del_advance(g, Ns_second, 2, max_dist, 
-                bub, 0, (u_int32_t)-1, &convex2);
+                bub, 0, (uint32_t)-1, &convex2);
                 l2 = min_thres + 10;
             }
 
@@ -6896,7 +6896,7 @@ ma_hit_t_alloc* reverse_sources, long long miniedgeLen, R_to_U* ruIndex)
                 kv_push(uint32_t, b_r, w);
 
                 aw = asg_arc_a(g, w);
-                min_edge = (u_int32_t)-1;
+                min_edge = (uint32_t)-1;
                 for (t = 0; t < nw; t++)
                 {
                     if(aw[t].del) continue;
@@ -6996,7 +6996,7 @@ ma_hit_t_alloc* reverse_sources, long long miniedgeLen, R_to_U* ruIndex)
         }
         //check the length
         if(to_del_l == 0 && asg_bub_end_finder_with_del_advance(g, 
-        b_f.a, b_f.n, max_dist, &bub, 0, (u_int32_t)-1, &sink)==1)
+        b_f.a, b_f.n, max_dist, &bub, 0, (uint32_t)-1, &sink)==1)
         {
             to_del_l = 1;
         }
@@ -7143,7 +7143,7 @@ ma_hit_t_alloc* reverse_sources, long long miniedgeLen, R_to_U* ruIndex)
             kv_push(uint32_t, b_r, w);
 
             aw = asg_arc_a(g, w);
-            min_edge = (u_int32_t)-1;
+            min_edge = (uint32_t)-1;
             for (t = 0; t < nw; t++)
             {
                 if(aw[t].del) continue;
@@ -7187,7 +7187,7 @@ ma_hit_t_alloc* reverse_sources, long long miniedgeLen, R_to_U* ruIndex)
                 kv_push(uint32_t, b_r, w);
 
                 aw = asg_arc_a(g, w);
-                min_edge = (u_int32_t)-1;
+                min_edge = (uint32_t)-1;
                 for (t = 0; t < nw; t++)
                 {
                     if(aw[t].del) continue;
@@ -7288,7 +7288,7 @@ ma_hit_t_alloc* reverse_sources, long long miniedgeLen, R_to_U* ruIndex)
         }
         //check the length
         if(to_del_l == 0 && asg_bub_end_finder_with_del_advance(g, 
-        b_f.a, b_f.n, max_dist, &bub, 0, (u_int32_t)-1, &sink)==1)
+        b_f.a, b_f.n, max_dist, &bub, 0, (uint32_t)-1, &sink)==1)
         {
             to_del_l = 1;
         }
@@ -7431,7 +7431,7 @@ ma_hit_t_alloc* reverse_sources, long long miniedgeLen, R_to_U* ruIndex)
             kv_push(uint32_t, b_r, w);
 
             aw = asg_arc_a(g, w);
-            min_edge = (u_int32_t)-1;
+            min_edge = (uint32_t)-1;
             for (t = 0; t < nw; t++)
             {
                 if(aw[t].del) continue;
@@ -7475,7 +7475,7 @@ ma_hit_t_alloc* reverse_sources, long long miniedgeLen, R_to_U* ruIndex)
                 kv_push(uint32_t, b_r, w);
 
                 aw = asg_arc_a(g, w);
-                min_edge = (u_int32_t)-1;
+                min_edge = (uint32_t)-1;
                 for (t = 0; t < nw; t++)
                 {
                     if(aw[t].del) continue;
@@ -7575,7 +7575,7 @@ ma_hit_t_alloc* reverse_sources, long long miniedgeLen, R_to_U* ruIndex)
         }
         //check the length
         if(to_del_l == 0 && asg_bub_end_finder_with_del_advance(g, 
-        b_f.a, b_f.n, max_dist, &bub, 0, (u_int32_t)-1, &sink)==1)
+        b_f.a, b_f.n, max_dist, &bub, 0, (uint32_t)-1, &sink)==1)
         {
             to_del_l = 1;
         }
@@ -31884,7 +31884,7 @@ int64_t max_tip, bub_label_t *b_mask_t, uint32_t is_trio, char *o_file)
     nopt.cov = coverage_cut; nopt.b_mask_t = b_mask_t;
     nopt.max_hang = max_hang_length; nopt.mini_ovlp = mini_overlap_length;
     ul_realignment_gfa(uopt, *sg, clean_round, min_ovlp_drop_ratio, max_ovlp_drop_ratio, 
-                                                                    asm_opt.max_short_tip, b_mask_t, ha_opt_triobin(&asm_opt), o_file, &nopt);;
+                                                                    asm_opt.max_short_tip, asm_opt.max_short_ul_tip, b_mask_t, ha_opt_triobin(&asm_opt), o_file, &nopt);;
     // ma_ug_t *iug = ul_realignment_gfa(uopt, *sg, clean_round, min_ovlp_drop_ratio, max_ovlp_drop_ratio, 
     //                                                                 asm_opt.max_short_tip, b_mask_t, ha_opt_triobin(&asm_opt), o_file);
     // asg_t *ng = gen_ng(iug, *sg, uopt, coverage_cut, ruIndex, 256);
diff --git a/gfa_ut.cpp b/gfa_ut.cpp
index 9f997d8..7892c67 100644
--- a/gfa_ut.cpp
+++ b/gfa_ut.cpp
@@ -14385,9 +14385,12 @@ void u2g_hybrid_detan_iter(ul_resolve_t *uidx, usg_t *ng, uint32_t max_ext, uint
     asg64_v b64, ub64; kv_init(b64); kv_init(ub64); 
     asg64_v tx = {0,0,0}, tb = {0,0,0}, *ob = NULL, *ub = NULL;
     ob = (in?(in):(&tx)); ub = (ib?(ib):(&tb)); ob->n = ub->n = 0;
+    // fprintf(stderr, "\n[M::%s::] asm_opt.is_low_het_ul::%u, max_ext::%u\n", 
+    //     __func__, asm_opt.is_low_het_ul, max_ext);
     // prt_usg_t(uidx, ng, "ng0");
     for (k = 0; k < clean_round; k++) {
         ncut += ug_ext_strict(ob, ub, uidx, ng, max_ext, &ff, &ng_occ, &i_idx, &b64, &ub64);
+        // if(asm_opt.is_low_het_ul) break;
         ncut += ug_ext_free(ob, ub, uidx, ng, max_ext, &ff, &ng_occ, &i_idx, &b64, &ub64, 48, 1);
         // prt_usg_t(uidx, ng, "ng_python");
         ncut += ug_ext_free(ob, ub, uidx, ng, max_ext, &ff, &ng_occ, &i_idx, &b64, &ub64, 16, 1);
@@ -14403,7 +14406,7 @@ void u2g_hybrid_detan_iter(ul_resolve_t *uidx, usg_t *ng, uint32_t max_ext, uint
     // prt_usg_t(uidx, ng, "ng.db");
     // ncut += ug_ext_free(ob, ub, uidx, ng, max_ext, &ff, &ng_occ, &i_idx, &b64, &ub64, 10);
     // ncut += ug_ext_strict(ob, ub, uidx, ng, max_ext, &ff, &ng_occ, &i_idx, &b64, &ub64);
-    // prt_usg_t(uidx, ng, "ng1");
+    // prt_usg_t(uidx, ng, "ng.db");
     if(ncut) {
         usg_cleanup(ng); usg_arc_cut_tips(ng, max_ext, 1, ub);
     }
@@ -16479,8 +16482,38 @@ void destroy_ul_resolve_t(ul_resolve_t *uidx)
     free(uidx);
 }
 
+static void clear_ma_hit_t_alloc(void *data, long i, int tid)
+{
+    ma_hit_t_alloc *src = (ma_hit_t_alloc *)data;
+    ma_hit_t_alloc *z = &(src[i]); uint32_t k;
+    for (k = 0; k < z->length; k++) z->buffer[k].del = 0;
+}
+
+static void reset_ma_sub_t(void *data, long i, int tid)
+{
+    sset_aux *s = (sset_aux *)data;
+    if(s->g && s->g->seq[i].del) s->cov[i].del = 1;
+    else s->cov[i].del = 0;
+}
+
+void renew_R_to_U(asg_t *ng, ma_hit_t_alloc* src, ma_hit_t_alloc* r_src, int64_t n_read, ma_sub_t *coverage_cut, 
+R_to_U* ruIndex, int max_hang, int min_ovlp)
+{
+    sset_aux s; memset(&s, 0, sizeof(s)); 
+    kt_for(asm_opt.thread_num, clear_ma_hit_t_alloc, src, n_read);
+    kt_for(asm_opt.thread_num, clear_ma_hit_t_alloc, r_src, n_read);
+
+    s.g = NULL; s.cov = coverage_cut;
+    kt_for(asm_opt.thread_num, reset_ma_sub_t, &s, n_read);
+
+    ma_hit_contained_advance(src, n_read, coverage_cut, ruIndex, max_hang, min_ovlp);
+
+    // s.g = ng; s.cov = coverage_cut;
+    // kt_for(asm_opt.thread_num, reset_ma_sub_t, &s, n_read);
+}
+
 void ul_realignment_gfa(ug_opt_t *uopt, asg_t *sg, int64_t clean_round, double min_ovlp_drop_ratio, 
-double max_ovlp_drop_ratio, int64_t max_tip, bub_label_t *b_mask_t, uint32_t is_trio, char *o_file,
+double max_ovlp_drop_ratio, int64_t max_tip, int64_t max_ul_tip, bub_label_t *b_mask_t, uint32_t is_trio, char *o_file,
 ul_renew_t *ropt)
 {
     uint64_t i; uint8_t *r_het = NULL; bubble_type *bub = NULL; ulg_opt_t uu;
@@ -16502,7 +16535,7 @@ ul_renew_t *ropt)
     // print_raw_uls_seq(uidx, asm_opt.output_file_name);
     // print_raw_uls_aln(uidx, asm_opt.output_file_name);
     ul_re_correct(uidx, 3); 
-    init_ulg_opt_t(&uu, uopt, clean_round, min_ovlp_drop_ratio, max_ovlp_drop_ratio, 0.55, max_tip, max_tip<<1, b_mask_t, is_trio);
+    init_ulg_opt_t(&uu, uopt, clean_round, min_ovlp_drop_ratio, max_ovlp_drop_ratio, 0.55, max_tip, max_ul_tip, b_mask_t, is_trio);
     // print_debug_gfa(sg, init_ug, uopt->coverage_cut, "UL.debug0", uopt->sources, uopt->ruIndex, uopt->max_hang, uopt->min_ovlp, 0, 0, 1);
     /**ul2ul_idx_t *u2o = **/gen_ul2ul(uidx, uopt, &uu, 0);
     // print_ul_alignment(init_ug, &UL_INF, 47072, "after-3");
@@ -16529,7 +16562,7 @@ ul_renew_t *ropt)
     asg_destroy((*(ropt->sg))); (*(ropt->sg)) = ng; 
     (*(ropt->src)) = R_INF.paf; (*(ropt->r_src)) = R_INF.reverse_paf;
     (*(ropt->n_read)) = R_INF.total_reads; (*(ropt->readLen)) = R_INF.read_length;
-    ma_hit_contained_advance((*(ropt->src)), (*(ropt->n_read)), (*(ropt->cov)), ropt->ruIndex, ropt->max_hang, ropt->mini_ovlp);
+    renew_R_to_U(ng, (*(ropt->src)), (*(ropt->r_src)), (*(ropt->n_read)), (*(ropt->cov)), ropt->ruIndex, ropt->max_hang, ropt->mini_ovlp);
     post_rescue(uopt, (*(ropt->sg)), (*(ropt->src)), (*(ropt->r_src)), ropt->ruIndex, ropt->b_mask_t, 0);
     // print_raw_uls_aln(uidx, asm_opt.output_file_name);
     // exit(0);
diff --git a/gfa_ut.h b/gfa_ut.h
index 8fc2274..5823e21 100644
--- a/gfa_ut.h
+++ b/gfa_ut.h
@@ -6,6 +6,7 @@
 typedef struct {
 	asg_t *g; 
 	ma_hit_t_alloc *src;
+    ma_sub_t *cov;
     R_to_U* ruIndex;
     int64_t max_hang; 
     int64_t min_ovlp; 
@@ -25,7 +26,7 @@ void asg_arc_cut_complex_bub_links(asg_t *g, asg64_v *in, float len_rat, float o
 uint32_t asg_cut_large_indel(asg_t *g, asg64_v *in, int32_t max_ext, float ou_rat, uint32_t is_ou);
 uint32_t asg_cut_semi_circ(asg_t *g, uint32_t lim_len, uint32_t is_clean);
 void ul_realignment_gfa(ug_opt_t *uopt, asg_t *sg, int64_t clean_round, double min_ovlp_drop_ratio, 
-double max_ovlp_drop_ratio, int64_t max_tip, bub_label_t *b_mask_t, uint32_t is_trio, char *o_file, ul_renew_t *ropt);
+double max_ovlp_drop_ratio, int64_t max_tip, int64_t max_ul_tip, bub_label_t *b_mask_t, uint32_t is_trio, char *o_file, ul_renew_t *ropt);
 void recover_contain_g(asg_t *g, ma_hit_t_alloc *src, R_to_U* ruIndex, int64_t max_hang, int64_t min_ovlp, int64_t ul_occ);
 void normalize_gou(asg_t *g);
 void prt_specfic_sge(asg_t *g, uint32_t src, uint32_t dst, const char* cmd);
diff --git a/hic.h b/hic.h
index 0f2ffda..a6bc80c 100644
--- a/hic.h
+++ b/hic.h
@@ -110,4 +110,6 @@ void set_utg_by_dis(uint32_t v, pdq* pq, asg_t *g, kvec_t_u32_warp *res, uint32_
 void dedup_hits(kvec_pe_hit* hits, uint64_t is_dup);
 void hic_analysis(ma_ug_t *ug, asg_t* read_g, trans_chain* t_ch, ug_opt_t *opt, uint32_t is_poy, kvec_pe_hit **rhits);
 spg_t *hic_pre_analysis(ma_ug_t *ug, asg_t* read_g, trans_chain* t_ch, ug_opt_t *opt, kvec_pe_hit **rhits);
+void prt_bubble_gfa_adv(FILE *fp, bubble_type *bub, const char* utg_pre, const char* bub_pre, const char* chain_pre);
+
 #endif