Initially, we focused on the canonical genes predisposed to HBOC, such as ATM, PALB2, Chek2, CDH1, PTEN, and TP53, but no PV was found, except for two missense variants in the ATM gene that is involved in DNA double-strand-break repair mechanisms and considered as a moderate-penetrance gene [39,40]. The gene discussed is CHEK2; the disease is acquired polycythemia vera.