The ichq mutation arose spontaneously in a colony of BALB/c mice at The Jackson Laboratory in 1989. The molecular basis for this mutation is a single nucleotide deletion of a G residue at sequence position 42 in exon 1 of the Cst6 gene. This results in a shift of the reading frame, resulting in a premature stop codon at amino acid position 20 of the encoded protein. In addition to the single nucleotide deletion, the mutant allele also carried two C-to-T SNPs in exon 1 at positions 33 and 40. Immunohistochemical analysis confirmed the predicted absence of cystatin M/E at the protein level in homozygous mice. (J:79976)