Put in worksheet code format column A as Marlett will put a check mark in column A, when you click in it, if there is something in the same row in another column. Click in the cell again and it will remove it, thanks to the board for help on this code.
Private Sub Worksheet_SelectionChange(ByVal Target As Range)
Dim rng As Range, cell As Range
If Selection.Cells.Count = 1 And _
Not Intersect(Selection, Columns(1)) Is Nothing And _
Not Intersect(Selection, ActiveSheet.UsedRange) Is Nothing Then
With Selection
If .Value = "a" Then
.ClearContents
Else: .Value = "a"
End If
End With
End If
End Sub