In humans, mutations in TINF2 (encoding TIN2), in ACD (encoding TPP1/ACD), and RTEL1 (regulator of telomere elongation helicase 1) (see below), are also causative of HHS, but with a drastic telomere shortening phenotype, linked to their role in telomere protection against DDR, independently of telomere length [17]. This evidence concerns the gene ACD and hypotrichosis 1.