SAMHD1 and Aicardi-Goutieres syndrome: The genetic dissection of a particular type I interferonopathy, Aicardi-Goutières syndrome, initially described as an early-onset progressive brain disease, identified mutations of the TREX1, RNASEH2A, RNASEH2B, RNASEH2C, SAMHD1, ADAR, and MDA5 genes as causal.