diff --git a/PORTING.md b/PORTING.md
index 70b67cc..04b28fa 100644
--- a/PORTING.md
+++ b/PORTING.md
@@ -123,6 +123,12 @@
 * If a HelloRequest is received while `SSL_write` has unsent application data,
   the renegotiation is rejected.
 
+### Lowercase hexadecimal
+
+BoringSSL's `BN_bn2hex` function uses lowercase hexadecimal digits instead of
+uppercase. Some code may require changes to avoid being sensitive to this
+difference.
+
 
 ## Optional BoringSSL-specific simplifications