APC and cancer: Extensive routine genetic diagnostics for the index patient (IV:3), including cancer gene panel sequencing, exome sequencing, and copy number analyses, were inconclusive (“Methods” section) and did not reveal a pathogenic variant in APC. Only a heterozygous, likely benign, synonymous APC variant (NM_000038.6:c.1959G>A (p.Arg653 = ))33,34 was found in individuals IV:3 and III:4 (Fig. 1).