Thymocyte enhancer E4t downstream of Cd4 (in intron 1 of Usp5) was replaced with an FRT site flanked hygromycin resistance gene cassette. Flp-mediated recombination removed the hygro cassette. Targeted ES cells carried the Rr94tm3.1Litt allele that targets the E4p Cd4 enhancer. (J:158962)