excel data validation for gender column M or F

I have a column named Gender where user have to fill either M or F. Is there any way except drop down so that I can restrict the users to enter only M or F in the cell?

In addition to Ford's solution above (which is exactly what I was going to give) it's worth pointing out that this gives you a drop down that you wanted to avoid, but just because it's there doesn't mean you need to use it. It will restrict the options a user can input, but they can do that by typing rather than mousing the drop down and still have the same effect.

BSB

Further to what Ford & BSB have said, if you uncheck the "In-cell dropdown" box then you wont get the dropdown.