ES cells containing the Rbp4tm2Zhel null allele that serves as an acceptor in recombination mediated cassette exchange were electroporated with a replacement vector and a cre recombinase expression vector. The replacement vector consisted of a wild-type human RBP4 cDNA and a FRT-flanked puromycin resistance gene flanked by a loxKMR3 and a loxP site. (J:220321, J:260683)