A recent study has found that babies who died from sudden infant death syndrome (SIDS) show lower levels of the brain chemical serotonin than babies who died from other causes. Serotonin regulates a baby's sleep, heart rate, and breathing; and abnormal levels of the neurotransmitter(神経伝達物質)could hamper an infant's breathing. The findings suggest that some babies have an underlying vulnerability to SIDS, and researchers hope there will one day be a test to identify those at risk.