Patients presenting with BM resident DTC populations that exhibit decreased expression of proliferation markers [e.g., marker of proliferation Ki-67, proliferating cell nuclear antigen (PCNA)], as well as the presence of CTCs up to 20+ years after initial treatment without overt tumor lesions argues for a reservoir of dormant cancer cells that can shed into circulation (Sauer et al., 2021). Here, PCNA is linked to neoplasm.