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. The gene discussed is AQP1; the disease is twin to twin transfusion syndrome.