CDKN1C and microcephaly: However, head sparing (normocephaly) was seen in 13% CDKN1C, 43% PLAG1, 42% IGF2 and 46% HMGA2. All monogenic causes had individuals with microcephaly (88% CDKN1C, 58% IGF2, 57% PLAG1 and 54% HMGA2).