A modified D3 ES cell line heterozygous for the Selptm1Hyn allele was targeted a second time. The exons encoding the signal peptide, lectin domain, and a portion of the epidermal growth factor domain were replaced by the insertion of a PGK-hygro cassette. Both Northern and RT-PCR analysis revealed an absence of normal message for both genes in extracts from cardiac and pulmonary tissue of homozygous mutant mice. This targeted mutation occurred in cis with the Selptm1Hyn mutation. Due to their genomic proximity, the two null alleles should propagate together through the offspring.