Place a formula into all selected cells?

I want to be able to select multiple cells (all within the same column if that matters) with the mouse and run a macro that will enter the value of cell a1 into all the selected cells. Is this possible? Thank you!

This Should Help:

Sub CellA1()
Range("").Select
Selection.FormulaR1C1 = "=R[-19]C"
End Sub

Awesome! Thank you!

Since that can be a dangerous button on a worksheet (you can't undo it), is there a way to only allow it to work if the selection is in a certain column? Thanks