NTRK1 and anhidrosis: However, similarly to SCN9A alleles, these HSN genes may influence more than one type of rare pathological pain conditions; a novel mutation in NTRK1 gene known to be responsible for HSN IV apparently causes HSN V [128], and other NTRK1 mutations and polymorphisms result in CIPA (congenital insensitivity to pain with anhidrosis) [129].