Data Validation within a worksheet
 

I wish to replace or add data dependent on a value in another cell.
Example:- value in cell A1=2, value in cell A2=4 or null. The value of cell
A2 must be 1,2, or 3 if A1=2
If A2 is invalid as in the example then I wish to replace the value or null
in A2 with 999.
Can anyone help me? Many thanks.