a, Shown at the top is the 2.8 mm cold biopsy forceps (Boston Scientific Corp.™) used for sampling tumor from some of the rectal cancers (RCs) used to derive tumoroids. Also shown is the first primary tumor sampled with this biopsy forceps stained with hematoxylin and eosin (H&E, second panel). The corresponding derived tumoroid, RC-MSK-008, in 3D culture is displayed by brightfield microscopy and H&E (lower 2 panels). Scale bars, 50 μm. b, Histopathologic staining of enterocyte markers (Alcian blue, CK20, and CDX2) of a primary resected rectal tumor (leftmost panels) and the corresponding tumoroid, RC-MSK-001, in 3D culture (right panels). Scale bars, 50 μm. c, The mutation landscape of 31 of the RC tumoroids is displayed (left panel) compared to an independent set of 287 RCs (right panel), both detected by MSK-IMPACT. The frequency of alterations in the RC tumoroids is noted with the type of genetic alteration (noted by color code). Displayed are the top 15 mutated genes observed in the derived tumoroids.