IL21 and neoplasm: Engineered Lister strain of VV with additional deletion of the VV 13.8-kDa N1L protein, a neurovirulence factor (named VVLΔTKΔN1L), and encoding IL-21 (VVLΔTKΔN1L-mIL-21) enhanced anti-tumour immune responses in murine models of colorectal cancer.