In this study, we found that proliferating spindle fibroblasts and myofibroblasts and varying amounts of collagenous fibrous tissues in fibromatosis-like undifferentiated carcinoma accounted for over 90% of the tumor content, and the CTNNB1 mutation was not detected in any of the seven patients. This evidence concerns the gene CTNNB1 and undifferentiated carcinoma.