Thank you so much! That works perfectly for Column A and Column B, but when I tried to edit the formula to expand the comparison range, it didn't work. I tried

=ISNUMBER(MATCH(A1,B:D,0))

I thought that the B:B portion of your original formula was the comparison range - that I would be comparing Column A to Columns B through D - but it didn't work. Is it necessary to create additional conditional formatting rules to compare other columns?

Thank you!