A PV was detected in 11 families, including six PV in APC, three homozygous PV in MUTYH, a homozygous PV in NTHL1, and a PV in POLE. When screening for mosaicism in blood and/or several adenomas, two additional pathogenic PV in APC were detected and were deemed to be likely representative of low-grade mosaicism. Here, APC is linked to acquired polycythemia vera.