ES cells containing a previously targeted Abcb1a allele (Abcb1atm1Bor) were targeted again with a vector replacing exons 3 and 4 of Abcb1b with a neomycin selection cassette. The vector contained a MluI linker 3' of the neo cassette that allowed detection of the Abcb1b disruption linked to the previous Abcb1a disruption by long-range restriction mapping. (J:39804)