To insert column of symbol or gap a nucleotide into a read in the Sanger Reads Editor, select a nucleotide or a gap in the corresponding location in the Chromatogram Editor select the nucleotide and press Area and switch to the editing mode by pressing Shift+I or , selecting Edit–>Insert character/gap context menu for switching the edit mode on. gap in the context menu, or selecting Actions->Edit->Insert character/gap in the main menu. While the selection cursor is blinking, press a character on the keyboard character that you want to insert. After that the edit editing mode is switched off. For insert another symbol press Shift+I again. You always can cancel the changes .
Note that if you press the '-' character on the keyboard while being in the editing mode, a gap is inserted. The other way to insert a gap is by pressing Space, see the Inserting Gap chapter.
You can always cancel the changes by pressing Ctrl+Z.