diff --git a/build_modeller.py b/build_modeller.py
index 11c6d09..1fbba11 100755
--- a/build_modeller.py
+++ b/build_modeller.py
@@ -25,6 +25,54 @@ class PDBModeler:
         self.sequence = self.fasta_file.stem 
         self.ali_file = self.fasta_to_ali()
 
+    @staticmethod
+    def find_non_dash_indices(seq):
+        start = next((i for i, c in enumerate(seq) if c != '-'), None)
+        end = next((i for i, c in enumerate(reversed(seq)) if c != '-'), None)
+        if start is not None and end is not None:
+            end = len(seq) - end
+        return start, end
+    
+    @staticmethod
+    def align_sequences(file: Path) -> Path:
+        fx = pyfastx.Fasta(file.as_posix(), build_index=True)
+        assert len(fx) == 2
+        seqs = [seq for seq in fx]
+        
+        # 确定哪条链需要裁剪
+        if seqs[0].seq.startswith('-') or seqs[0].seq.endswith('-'):
+            trim_index = 0
+        elif seqs[1].seq.startswith('-') or seqs[1].seq.endswith('-'):
+            trim_index = 1
+        else:
+            # 如果两条链都不需要裁剪，就直接返回原文件
+            return file
+
+        start, end = PDBModeler.find_non_dash_indices(seqs[trim_index].seq)
+        
+        # 根据确定的裁剪位置裁剪两条链
+        trimmed_seqs = []
+        for seq in seqs:
+            trimmed_seq = seq.seq[start:end]
+            trimmed_seqs.append(SeqRecord(Seq(trimmed_seq), id=seq.name, description=""))
+
+        # 选择没有'-'的序列
+        selected_seq = None
+        for seq_record in trimmed_seqs:
+            if '-' not in seq_record.seq:
+                selected_seq = seq_record
+                break
+
+        assert selected_seq is not None, "no sequence without '-' found"
+        assert not selected_seq.seq.startswith('-') and not selected_seq.seq.endswith('-'), "selected sequence should not start or end with '-'"
+
+        # Write the selected sequence to a new FASTA file using Biopython
+        new_fasta_file = file.with_suffix('.selected.fasta')
+        with open(new_fasta_file, 'w') as output_handle:
+            SeqIO.write([selected_seq], output_handle, "fasta")
+
+        return new_fasta_file
+
     def make_model(self): # 单模板建模
         print("***************************************************")
         print("md_level ====", self.md_level)