B2 Microglobulin (B2M) is a ubiquitous, 12kD, secreted, non-glycosylated protein required for cell surface expression and noncovalent assembly of MHC Class I molecules and CD1 cell surface glycoproteins. Mature rat B2M is a 99aa peptide containing one C1-type lg-like domain (aa22-116). In humans, B2M is known to dissociate from the MHC complex and circulate as full-length and N-terminal truncated peptides of 93, 91, and 90aa. Mature rat B2M is 86% and 75% identical to the corresponding mouse and human protein sequences, respectively.