This mutation was discovered in an ENU mutagenesis screen. It comprises a T-to-A transversion at nucleotide 93085 of the genomic sequence, within an intron, which results in the insertion of the 5-base pair sequence ATCAG following nucleotide 1791 of the mRNA. This in turn causes a translational frame shift, introducing 26 new amino acids before a premature termination codon. No CD45/PTPRC protein is visually detectable on Western blot analysis of mutant thymocytes, and expression of CD45 is reduced on CD19+ B cells of homozygous mutant mice.