SNPs are distributed in the human genome in a non-random fashion [53], and double mutations are considered secondary events in the context of an already existing SAP, as is exemplified with the Andersen-Tawil syndrome, where two missense mutations in the KCNJ2 gene are located on the same allele [54]. The gene discussed is KCNJ2; the disease is Andersen-Tawil syndrome.