Hi,

May be with double click ...

Private Sub Worksheet_BeforeDoubleClick(ByVal Target As Range, _
Cancel As Boolean)
With Target
If .Cells.Count 1 Then Exit Sub
If Intersect(.Cells, Me.Range("B:B,D:D")) Is Nothing Then Exit
Sub
If .Value = "" Then
.Value = "X"
Else
.Value = ""
End If
End With
End Sub


HTH
Cheers
Carim