Beta catenin is a transcriptional co-regulator and an adapter protein for cellular adhesion; it comprises part of the Wnt signaling pathway and intracellular levels of beta-catenin are regulated by its phosphorylation, ubiquitination and proteosomal degradation. Accumulation of nuclear beta catenin can lead to a tumoral phenotype and oncogenic transformation in a variety of solid tumors. Various oncogenic mutants of beta catenin have been found in different tumor types which alter its degradation, leading to its accumulation and promoting tumor growth. Mutations in exon 3 of CTNNB1 result in stabilization of a protein that resists degradation, leading to nuclear accumulation of β-catenin, have been described in endometrioid endometrial carcinoma. The reported frequency of CTNNB1 mutations in endometrioid endometrial carcinoma ranges from 14–44%. However, these mutations are not described previously in endometrial sarcomas. Of note, CTNNB1 mutations are highly common in desmoid fibromatosis.