diff --git a/crisprbact/off_target.py b/crisprbact/off_target.py
index 89a5a648b0a21e56c1f055b7e619a26743760805..176c25ab4538d5b5dc263b77779b0981fb133a83 100644
--- a/crisprbact/off_target.py
+++ b/crisprbact/off_target.py
@@ -24,7 +24,7 @@ def get_off_target_pos(guide, records, seed_size):
     if records is not None:
         offs = iter([])
         for rec in records:
-            sequence_str = str(rec.seq)
+            sequence_str = str(rec.seq).upper()
             seqid = rec.id
             offs_plus = re.finditer(guide[-seed_size:] + "[ATGC]GG", sequence_str)
             offs_plus = gen_extract_off_target_strand_plus(