HYDIN2 and microcephaly: While it was speculated that HYDIN2 played an important role in the reciprocal macrocephaly/microcephaly phenotype associated with 1q21.1 duplications/deletions [22], we have identified rearrangement patients with head size abnormalities lacking altered genomic dosage of HYDIN2. It is plausible that altered expression or point mutations effectively disrupt HYDIN2 in these individuals or, alternatively, that HYDIN2 dysfunction does not contribute to their head circumference phenotypes.