Hereditary deficiency of the second component of complement (C) was the first human inherited C component deficiency state described (1). It also appears to be the most common C component deficiency thus far studied with a gene frequency of 1% (2, 3). The genetic basis for this deficiency is not as yet known. In contrast, the basis for the genetic deficiencies of C3 and C6, which occur much less frequently, have been established. These deficiencies have been shown to be due to inheritance of silent or null genes allelic with the structural genes (4, 5). The approach used in those studies was to identify families with the deficiency that had informative electrophoretic variants of the proteins. Study of the inheritance of the structural variants and the serum levels of C components in such families provided evidence for the silent or null gene mechanism.

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