The adaptation of an in vivo CTL assay originally developed by Barchet et al.[7] to the functional study of in vivo generated virus-specific CD4+T cells by Jellison et al.[8] therefore constitutes an experimental advance that eschews possible artifacts that may arise through the use of in vitro generated CD4+CTL (e.g., skewing of T cell functionalities through unphysiological stimulation protocols) and/or the specific constraints of in vitro CTL assays (e.g., the preferential use of tumor rather than primary cells as targets). The gene discussed is CD4; the disease is neoplasm.