Caldesmon (CALD1) is a calmodulin binding protein that regulates smooth muscle contraction. It plays an inhibitory role in the “on” and “off” activity transition of smooth muscle thin filaments. In cancer, overexpression of caldesmon may be predictive of progression in non-muscle-invasive bladder cancers. In immunohistochemistry of normal tissue, caldesmon is positive in smooth muscle and in myoepithelial cells of the breast. In tumors, it is positive in those of smooth muscle origin such as uterine smooth muscle tumors, angioleiomyomas, leimoyomas and leiomyosarcomas. It is also positive in glomus tumors, GIST, and some endometrial stromal tumors. It is useful for distinguishing smooth muscle cells from myofibroblastic cells. References: Oncotarget. 2015 Nov 24;6(37):40370-84, PMID: 26430961; The Journal of Biological Chemistry. 2017 Oct 12;283:47-56, doi: 10.1074/jbc.M706771200.