obsolete xm system true HGNC:12813 OBSOLETE. Berg and Bearn (1966, 1966) discovered an X-linked serum protein type by means of heteroantiserum made specific by absorption. Since the group-specific antigen appears to be located in the alpha-2-macroglobulin of serum, the name Xm was assigned to the system. The distribution of phenotypes in families and in populations was consistent with X-linkage. Obsoleted as it represents a trait or is a legacy entry MONDO:0044266