This study identifies a family meeting the criteria for FCCTX where a germline BRCA1:c.2681_2682delAA p.Lys894ThrfsTer8 pathogenic variant and a germline RNF43:c.988 C>T p.Arg330Ter pathogenic variant co-segregated with CRC in four carriers, one of whom was confirmed to meet the WHO2019 diagnostic criteria 1 for SPS. Here, RNF43 is linked to colorectal carcinoma.