Hypotonia (especially axial) can be present in 22 (81.5%) conditions: AP3B2, ATP1A3, CACNA1E, CDKL5, DNM1, FHF1, GABRB2, GABRB3, GABRG2, GRIA2, GNAO1, KCNQ2, PCDH12, PURA, RHOBTB2, SCN2A, SCN8A, SLC13A5, STXBP1, SYNJ1, VAMP2, and WWOX; microcephaly in 12 (44%): AP3B2, DNM1, FHF1, GABRA1, GABRB2, GRIA2, GNAO1, PCDH12, SCN8A, and SLC13A5 (absent in neonatal-onset cases), postnatal in ATP1A3, head growth deceleration in CDKL5. On the contrary, macrocephaly is rare (CACNA1E) [42]. This evidence concerns the gene STXBP1 and Hypotonia.