STX1B and neurodevelopmental disorder: Since the initial discovery of STXBP1, genetic variants in other genes encoding presynaptic proteins have been found to cause neurodevelopmental disorders, including genes that encode components of the presynaptic fusion machinery such as SNAP25, VAMP2 or STX1B.8,17-19 Synaptic vesicle recycling also represents a prominent disease mechanism, and we have previously identified disease-causing variants in DNM1 and AP2M1 in neurodevelopmental disorders.20,21