Previous evidence indicates that mutations in the GALNT12 gene might cause a fraction of the unexplained familial colorectal cancer (CRC) cases: GALNT12 is located in 9q22-33, in close proximity to a CRC linkage peak; and germline missense variants that reduce the enzymatic activity of the protein have been identified in CRC patients, some of them with familial CRC history. We hypothesized that mutations in GALNT12 might explain part of the high-risk families grouped as familial CRC type X (fCRC-X), that is, Amsterdam-positive families with mismatch repair proficient tumors. We sequenced the coding regions of the gene in 103 probands of fCRC-X families, finding no functionally relevant mutations. Our results rule out GALNT12 as a major high CRC susceptibility gene. Additional studies are required to provide further evidence about its role as a moderate/low susceptibility gene in familial aggregation of cancer.