This spontaneous mutation, discovered in the C57BL/6J colony at the Medical Research Council, Harwell, UK, has been identified as a point mutation (C-to-A) in exon 5 that introduces a new acceptor splice site. It is located at position chr2:83295206 (GRCm38) and creates a new CAG splice acceptor site from CCG. This acceptor is used in splicing, removing the first 39 nucleotides of exon 5 from the mRNA and 13 amino-acids from the encoded peptide.