MUC21 and rectum adenocarcinoma: MUC16 mutations were found in 26.7% of the mucinous rectal cases and one of the MSI-H mucinous cases also had an SNV in MUC21. The only mutations found in mucin genes in the TCGA rectal adenocarcinoma NOS cohort was a 9.5% mutation rate in the MUC1 gene.