Sequence
Information
Select a sequence and choose Edit
--> Get Info 
The Sequence Information menu lets you set properties of individual
sequences.
Character protections - By
default, only alignment gaps may be inserted or deleted. Check boxes
set which kind of characters are protected from deletion: Alignment
gaps (-), Ambiguous characters (eg. y, r, n for DNA) or Unambiguous
characters (eg. AGCTU for DNA/RNA). The Backspace
button deletes characters to the left of the cursor. The Delete button
deletes characters to the right of the cursor.