Conditional Entry into Excel

I want the entry of a value into Cell B1 be dependant on a value having been entered into cell A1.

All other cells are locked. I have 5 cells (A1:E1) into which data of different type must be entered. The condition is that in order to enter a value into a cell (eg. B1) that a predefined cell(eg. A1) already contain a value (which has already been data validated), thus creating a sequential entry of data and preventing a cell from being 'missed out'.

Tried Data Validation but without success.