Provided herein are novel organoid culture media, organoid culture systems, and methods of culturing tumor organoids using the subject organoid culture media. Also provided herein are tumor organoids developed using such organoid culture systems, methods for assessing the clonal diversity of the tumor organoids, and methods for using such tumor organoids, for example, for tumor modelling and drug development applications. In particular embodiments, the tumor organoid culture media provided herein is substantially free of R-spondins (e.g., R-spondin1).