In this study, ER+ BC monocultured spheroids (herein referred as MCF7 or ZR-75 spheroids) and organoids composed of primary ASCs co-cultured with ER+ cell lines (herein referred to as MCF7 or ZR-75 organoids) were 3D cultured through a previously developed indexable microfluidic droplet generator and trapping array coupled with a thiol acrylate (TA) hydrogel scaffold.39,40 This allowed for the culture of ∼450 spheroids or organoids in a single device with three devices being run in parallel to study intra-tumor heterogeneity. Here, ESR1 is linked to breast cancer.