show Abstracthide AbstractHuman embryonic stem cells (hESCs) provide a platform for understanding mechanisms underlying diseases and finding ways to treat them. Trilateral retinoblastoma (TRb) is a condition in which retinoblastoma, the most common congenital intraocular malignancy, is associated with an intracranial neural tumor. This mostly fatal disease is caused by biallelic inactivation of the retinoblastoma-1 (RB1) gene, transcribing pRB. Using CRISPR-Cas9 we generated RB1-null hESCs, an embryonic lethal condition. These cells display distinct gene expression patterns with alterations in pRB targets, and mitochondrial phenotypes similar to those of poorly differentiated retinoblastomas. RB1–/– hESC produce extremely large teratomas, with neural expansions similar to those of TRb tumors. Analysis of these tumors revealed a potentially novel role of pRB in ectoderm differentiation, acting through ZEB1. Lastly, RB1–/– cells are significantly sensitive to carboplatin, a chemotherapy used to treat TRb, suggesting our model can be used to screen novel treatments for the disease. Overall design: Analysis of RB1–/– human embryonic cell lines and teratomas initiated from them