Multiple Common Properties of Human β 2 -Microglobulin and the Common Portion Fragment Derived from HL-A Antigen Molecules

11,000-Dalton common portion fragments derived from HL-A antigen molecules were isolated and found to have a significantly high homology to beta(2)-microglobulin in amino-acid composition. Common portion fragments are also very similar to beta(2)-microglobulin with respect to molecular size, charge, and distribution in tissues. Moreover, both are found in the spent culture media of human cell lines and in human plasma and urine. Thus it appears that beta(2)-microglobulin may well be the same substance as the common portion fragment of HL-A antigen molecules.