RNA polymerase II is a protein encoded by the POLR2A gene in humans. In coordination with a wide array of interacting partners, RNA polymerase II has been shown to participate in the regulation of the cell cycle, transcription, and splicing, all of which are important for ALL growth. Here, we investigated the RNA Pol II polymerase interactome, focusing on the splicing-related apparatus to identify the splicing factors involved in co-transcriptional splicing, which is important for ALL cell growth.