ES cells containing the Kpnb1Gt(pU-hachi)1Arak allele were targeted with a cre plasmid and a vector containing Kpnb1 cDNA, resulting in the replacement of the IRES-betageo within the gene trap vector with Kpnb1 cDNA, and PGK-puro. Mutants expressed both the endogenous and inserted sequences. (J:105350)