FLT1 and twin to twin transfusion syndrome: In 2005, AQP1 was known to be the possible candidate gene responsible for causing TTTS; later, four genes, i.e., AVPR1A FLT1, NRXN3, and NTRK3—related to vasoconstriction, angiogenesis, synapse formation, cardiac, and neurological development, respectively—were confirmed to be related to TTTS using microarray.