HSPG2 and Smith-Lemli-Opitz syndrome: The neurotrophin signaling pathway includes neurotrophins and their second messenger systems such as the MAPK pathway, PI3K pathway, and PLC pathway, which have been identified by others [40], [41] as important for neural development, learning and memory, and syndromic ASDs such as tuberous sclerosis and Smith-Lemli-Opitz syndrome.