diff --git a/README.md b/README.md
index 6494c25..c823294 100644
--- a/README.md
+++ b/README.md
@@ -76,6 +76,7 @@ Optional environment variables:
 * `GATEWAY_EXTRA_SCRIPTS` - JSON array of script paths, will be embedded into each page and forwarded with `--scripts` to cellxgene server
 * `GATEWAY_ENABLE_ANNOTATIONS` - Set to `true` or to `1` to enable cellxgene annotations. 
 * `GATEWAY_ENABLE_BACKED_MODE` - Set to `true` or to `1` to load AnnData in file-backed mode. This saves memory and speeds up launch time but may reduce overall performance.
+* `S3_ENABLE_LISTINGS_CACHE` - Set to `true` or to `1` to cache listings of S3 folders for performance. Can be overridden by setting `filecrawl.html?refresh=true` query parameter.
 
 If any of the following optional variables are set, [ProxyFix](https://werkzeug.palletsprojects.com/en/1.0.x/middleware/proxy_fix/) will be used.
 * `PROXY_FIX_FOR` - Number of upstream proxies setting X-Forwarded-For