MAPK1 and neoplasm: To study the effect of CTLs recognizing endogenous antigens, we employed a single cell-derived clone (CMS5a1) from CMS5 cells that express spontaneously mutated extracellular signal-regulated kinase (ERK) as the endogenous tumour antigen29 and a CMS5a1-derived single cell clone expressing DN of IFN-γR (CMS5a1γRDN).