In addition to high 5-HT1A binding in SIDS infants, a unique, unexpected feature of the current study was the high proportion of infants born prematurely, which drove our main findings, and may have uncovered a unique, and not previously well appreciated, subset or pathophysiological endotype of SIDS in premature infants with serotonin abnormalities distinct from SIDS infants born at term. Here, HTR1A is linked to sudden infant death syndrome.