MRTFB and microcephaly: We suggest that disruption of the MKL2:SRF gene transcription pathway, perhaps through diminished MKL2 heterodimerization with SRF due to the point mutation in the basic domain followed by the lack of critical CArG binding domains removed by the private deletion, accounts for extreme microcephaly in the affected infants and support this conclusion with the observation of reduced PCTAIRE1 gene and protein expression.