The tuberous sclerosis complex (TSC) family of tumor suppressors, TSC1 and TSC2, function together in an evolutionarily conserved protein complex that is a point of convergence for major cell signaling pathways that regulate mTOR complex 1 (mTORC1). Mutation or aberrant inhibition of the TSC complex is common in various human tumor syndromes and cancers. The discovery of novel therapeutic strategies to selectively target cells with functional loss of this complex is therefore of substantial clinical relevance to TSC and sporadic cancers. We developed a CRISPR-based method to generate homogenous mutant Drosophila cell lines. By combining TSC1 and TSC2 mutant cell lines with RNAi screens against all kinases and phosphatases, we identified synthetic interactions with TSC1 and TSC2. Knockdown of three candidate genes (mRNA-cap, Pitslre and CycT; orthologs of RNGTT, CDK11 and CCNT1 in humans) reduced the population growth rate of both Drosophila TSC1 and TSC2 mutant cells but not that of wild-type cells. Moreover, knockdown of all three genes displayed similar selective effects in mammalian TSC2-deficient cell lines, including human tumor-derived cells, illustrating the power of this cross species screening strategy to identify potential drug targets.