MAPK1 and microcephaly: While downregulation of RAS/ERK can lead to a thicker cortex from increased proliferation and an increased neuronal count [86], loss of Erk2 can induce such strong G1/S dysregulation that mice cells miss the time window of neurogenesis (E14.5–E16.5), resulting in fewer neurons, precocious astrogenesis, and potentially microcephaly [95].