IFIH1 and Aicardi-Goutieres syndrome: Gain-of-function mutations of IFIH1, encoding the cytosolic double-strand RNA receptor MDA5, results into a heterogeneous spectrum of phenotypes, and chilblain-like lesions, SMS, AGS and SMS/AGS syndromes overlap, while neurologic features and clinical non-penetrance have been reported within the same family [95,97,109].