GP9 and sudden infant death syndrome: While this would appear to argue against platelet activation as a cause of higher plasma and serum 5-HT in SIDS and the decrease in platelet surface GPIX in SIDS subjects, under some physiologic conditions the activation-dependent decrease in platelet surface GPIX does not correlate with platelet dense granule release (as reflected by increased platelet surface CD63 and histamine release)44 or platelet alpha granule release (as reflected by increased platelet surface P-selectin)45.