Alignment of the amino acid sequence of β-casein as deduced from the cDNA obtained from C57BL/6J and SEG/Pas. Peptides sequences are split into blocks of amino acid residues to visualize the exonic modular structure of the protein as deduced from known splice junctions of the Mus musculus sequence. Red amino acids in C57BL/6J sequence indicate the amino acid substitutions in SEG/Pas sequence. A* depicts a conservative mutation in the corresponding codon (GCC-> GCT in M. spretus). The pink vertical arrow indicates the deletion of a Q residue arising in the 6th exon, during the course of the splicing process in C57BL/6J and SEG/Pas mouse strains.