Serum substance (Ss), a serum protein, is known to be linked to the major histocompatibility locus of the mouse. Until recently the function of Ss was unknown but several lines of evidence, i.e. serologic, functional, and structural suggest Ss is murine C4.
The Ss protein has been isolated in pure form from murine ascites fluid and has been shown to have functional C4 activity. Purification was monitored by antigenic reactivity with a specific antiserum to Ss as well as with a hemolytic assay using genetically C4-deficient guinea pig serum.