Condensates regulate transcription by selectively compartmentalizing biomolecules, yet the rules of specificity and their relationship to function remain enigmatic. To identify rules linked to function, we leverage the genetic selection bias of condensate-promoting oncofusions. Focusing on the three most frequent oncofusions driving translocation renal cell carcinoma, we find that they promote the formation of condensates that activate transcription by gain-of-function RNA polymerase II partitioning through a shared signature of elevated π and π-interacting residues and depletion of aliphatic residues. This signature is shared among a broad set of DNA-binding oncofusions associated with diverse cancers. We find that this signature is necessary and sufficient for RNA polymerase II partitioning, gene activation, and cancer cell phenotypes. Our results reveal that dysregulated condensate specificity is a shared molecular mechanism of diverse oncofusions, highlighting the functional role of condensate composition and the power of disease genetics in investigating relationships between condensate specificity and function.