RAC1 and microcephaly: Notably, since RAC1 activates PAK, which assists in the MEK/ERK pathway via phosphorylation of c-RAF and MEK1 [99], RAC1 variants implicated in developmental delay during neuronal maturation exhibited dominant-negative properties causing microcephaly, diminished neuronal proliferation, and cerebellar anomalies when overexpressed in zebrafish [88].