A loxP site was inserted upstream of the promoter region and an F3 site, attB/P site, HygR cassette, F3 site and loxP site were inserted downstream of exon 1. Flp mediated recombination removed the selection cassette leaving the gene floxed. Targeting was done in ES cells carrying Bcl2a1atm1Tac and Bcl2a1dtm1Tac