ESR1 and neoplasm: Immunohistochemical staining showed tumor cells positive for desmin (Fig. 3B) and smooth muscle actin, as well as caldesmon, calponin, estrogen receptor (ER), and progesterone receptor (PR); but it showed negative for cluster of differentiation (CD) 34 and CD117, consistent with a diagnosis of pelvic retroperitoneal leiomyoma.