Thanks Colin,
It works
I'm going to use this rule for a range of cell $A$2:$E$10, where $A$1:$E$3 is a header row.
I assigned the formula =
OR(ISNUMBER(FIND(Letters,A1))) to the range - and it's working.
I don't understand that. FIND(find text1, within text2) - the function returns position number of text2 in text1. My A1 contains some text without T, F, A, and when I enter a, f, or t in $A$2:$E$10 I'm getting formated text.