Serial cryocut muscle cross-sections from cyclists after the pre-competitive season demonstrate immuno-localization of different ERV env genes and their receptors, transcription factors (pCREB-Ser133, PPARγ, and RXRα). An antibody recognizing keratins from the skin was used as a negative control for skeletal muscle, as well as a negative (neg.) control without primary antibodies. Long-Term Endurance Exercise in Humans Stimulates Cell Fusion of Myoblasts along with Fusogenic Endogenous Retroviral Genes In Vivo: Frese S, Ruebner M, Suhr F, Konou TM, Tappe KA, Toigo M, Jung HH, Henke C, Steigleder R, Strissel PL, Huebner H, Beckmann MW, van der Keylen P, Schoser B, Schiffer T, Frese L, Bloch W, Strick R - PloS one (2015). Not altered. CC.