Matrix-producing carcinoma (MPC) of the breast is a rare variant of the uncommon group of malignancies categorized as metaplastic breast carcinomas with heterologous elements. The major criterion for a diagnosis of MPC is the presence of invasive breast carcinoma with the direct transition to a cartilaginous or osseous stromal matrix without an intervening spindle cell component. The cellular origin of MPC remains unclear. It has been suggested that tumor cells in MPC have combined epithelial and mesenchymal features. Several reports have suggested that the tumor cells originate from myoepithelial cells. The prognosis of patients with MPC was originally described as similar to invasive mammary carcinomas of no special type (NST) of the same stage, but a more recent study has shown a worse prognosis than same-stage NSTs.