RB1 and neoplasm: Immunocytochemistry and double immunofluorescence staining of pRB and GrB showed that endogenous GrB protein (Endo-GrB) was accumulated in the pRB+ (in Tc-free medium), but not in the pRB− (in medium containing 0.5 μg ml−1 of Tc) MDA-MB-468 tumour cells (Figure 3A).