Desmoid fibromatosis (DF) is a rare, locally aggressive, nonmetastasizing fibroblastic/myofibroblastic tumor with a tendency to recur and an unpredictable clinical course. A "wait-and-see" policy is the new standard of care. DF are characterized by activating alterations of the wnt/β-catenin pathway: CTNNB1 or adenomatous polyposis coli gene (APC) mutations (these mutations being mutually exclusive). Desmoid-type fibromatosis of the breast (DFB) is rare with an incidence of 0.2% of breast tumors. The diagnosis of DFB is difficult, as it must be distinguished from metaplastic carcinoma and other spindle cell lesions. Sequencing of 128 DFB identified a lower rate of CTNNB1 mutations using Sanger (65.6%) or Sanger+next-generation sequencing (77.7%) and a higher rate of APC mutations (11.8%) than in all-site DF. By excluding patients with familial adenomatous polyposis (n=2), the rate of APC mutations in DFB was high (10.7%). The distribution of CTNNB1 mutations in DFB was different from all-site DF, with a higher rate of T41A (68.9%), a lower rate of S45F (5.7%), and a similar rate of S45T (12.6%). By combining the 2 molecular techniques in a 2-step manner (Sanger, then next-generation sequencing), we increased the detection rate of CTNNB1 mutations and lowered the rate of wild-type tumors from 34.4% to 9.8%, therefore improving the diagnosis of DFB. The identification of the exon 3 CTNNB1 mutation in breast spindle cell lesions is a highly specific tool for the diagnosis of DFB, in addition to extensive immunohistochemical analysis. Our study also underlines the importance of APC in DFB tumorigenesis. These findings have significant implications for patient care and management.