RBBP8 and Primary microcephaly: As the above factors suggested that CtIP defects might yield SS, we examined DNA samples from two unrelated microcephalic families that both map to the SCKL2 locus: the original SCKL2 family [3] and a family diagnosed with a Seckel-like type of congenital microcephaly termed Jawad syndrome [14] (see Figure S1A and S1B).