Conditional Formatting cells in a large grid....

Hi

I have the attached spreadsheet which makes the question easier to understand. Basically when you read across and down rows and columns, I'd like the intersecting cell to turn red if the horizontal and vertical row columns letters are different. So in the attached B5 is red because B1=A and A5 is B. Where the intersecting cells match in values, the cells will stay white.

I can create a formula to conditionally format one cell to red: =A2<>B1

but i have a large grid and it would mean re-creating each formula for each cell. Any idea on how to expand it to the whole range?