Inherent in many in vitro assays, one of the strengths of the spheroid assay is the relative ease of focused manipulation to aid in answering specific hypotheses. Namely, prior to the paired co-culture one of the cell types can be genetically altered by retro- or lenti-viral constructs, including fluorescence (gain-of-function) or by shRNA/siRNA knockdown or CRISPR/Cas9 (loss-of-function). Additionally, the sensitivity or resistance to a particular pharmacological or radiation treatment approach can be tested. We and others have observed marked differences in treatment response and viability of tumor cells growing in the presence/in contact with EC and vice versa