PRF1 and neuroblastoma: To determine if the augmented production of perforin, granzyme B and IFNγ would endow Cbx3/HP1γ-insufficient CD8+ T cells to better induce apoptosis than wt cells, in vitro-activated CD8+ and CD4+ T cells were co-cultured with target neuroblastoma NB-9464 tumor cells35, and tumor-cell apoptosis assessed by measuring cleaved/activated caspase 3 (CC3) production.