The detection of an inherited X-linked serum system in man, disclosed by a heteroantisenim made specific by absorption, is described. These studies suggest that the antigen, demonstrated by the specific antiserum, resides in the α2-macroglobulin fraction of serum. The system has been named the Xm system, where X refers to the localization of the gene on the X chromosome, and the m indicates that the antigen is part of the α2-macroglobulin fraction.

The distribution of phenotypes in unrelated individuals from 4 populations, as well as studies performed in families are consistent with an X-linked dominant mode of inheritance. One Xm(a-) daughter was found in a family where the father was Xm(a+). This finding is discussed with particular reference to the possible influence of sex and age on the development of the phenotype.

This common X-linked marker is likely to prove useful in mapping the human X chromosome.

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