The three selectin genes, Sele, Sell, and Selp, are found within a 300 kb genomic region. ES cells heterozygote for the Selptm1Hyn and Seletm1Hyn null alleles in cis to each other were targeted a third time. A PGK-puro cassette was used to delete a 2.1 kb region containing the exons encoding the lectin domain and most of the epidermal growth factor domain. Flow cytometric analysis showed an absence of the L-selectin protein but not the two other selectin proteins in leukocytes isolated from homozygotes. Thus, this targeted mutation occurred in trans with the other two null mutations and were subsequently bred apart.