The three selectin genes, Sele, Sell, and Selp, are found within a 300 kb genomic region. In order to create a triple knockout, 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 2.1kb region containing the exons encoding the lectin domain and most of the epidermal growth factor domain. Flow cytometric analysis showed an absence of all three encoded proteins in leukocytes isolated from homozygotes. This targeted mutation occurred in cis with the other two null mutations. Due to their genomic proximity, the three null alleles should propagate together through the offspring.