The two mouse POT1 proteins are highly homologous and can associate with telomeric DNA but while the Pot1b knock-out (KO) mouse is viable and does not present phenotypes except when combined with telomerase deficiency [35,36], Pot1a deletion in mice is embryonic lethal hampering the study of POT1a role in cancer development [34]. The gene discussed is POT1; the disease is cancer.