As the tumour regresses the number of immune cells (particularly CD8+ T cells) increases, peaking when the tumour is in an early regression phase.56–58 NK cells and cytotoxic T cells from dogs vaccinated with CTVT cells will kill CTVT cells from stationary and regressing tumours in vitro, indicating that these cells contribute to tumour regression.59 This evidence concerns the gene CD8A and neoplasm.