These mutations indicate functional haploinsufficiency where the homozygous forms are embryonically lethal. Erasure of the lactosylceramide synthase by quality focusing on is embryonically deadly. I cells are embryonically derived from the neural crest. Embryonically the gastrosplenic ligament is derived from the dorsal mesogastrium. A homozygous loss-of-function of Delta 1 ("Dll1") is embryonically lethal.