This phenotypic mutant was identified in an ENU mutagenesis screen at the Baylor College of Medicine. The molecular lesion is a c.487T>C transition mutation (p.S163P change in the encoded protein) in exon 5 for the l11Jus4 allele. This non-conservative amino acid substitution has a high probability to alter protein function, as serines easily form hydrogen bonds with polar substrates, while prolines are rarely found in active sites. In addition, an endogenous C57BL/6J non-synonymous SNP rs13468707 (c.538A>G transition; p.I180V) is present in exon 6. This is the equivalent of well-documented human SNP rs2820949 and leads to a very conservative amino acid substitution. Thus, l11Jus4 homozygotes harbor two coding changes in this gene: an ENU-induced point mutation and an additional spontaneous missense mutation that is also present in the C57BL/6J parent strain.