Benign metastasizing leiomyomatosis smooth muscle cells express estrogen and progesterone receptors on immunohistochemical staining, but are usually negative for Ki-67, which means that the metastatic nodes have a low proliferative index.48 Immunohistochemical staining helps differentiate BML from leiomyosarcoma (BML has a low Ki-67 proliferative index48-50). This evidence concerns the gene MKI67 and leiomyosarcoma.