B2m^b

The b variant is runs faster than the a variant on SDS PAGE and was found to have an alanine at amino acid position 85. This allele is found in the C57BL strain family including C57BL/6, C57BL/10, C57L/J, C57BR/cdJ, as well as CAST/Ei, but not in C58/J, which has the a variant. (J:6520, J:7014)