RTTN and microcephaly: The clinical phenotype of MSSP, however, is particularly heterogeneous (Supplementary Table S1) (7) and it is known that microcephaly may be lacking in affected individuals: about 81% of the individuals are microcephalic at birth, and in total 86% have secondary microcephaly, which means that normal head circumference at birth is not an exclusion criterion for RTTN mutations (6).