U2AF1 and cancer: Three specific components of complex A have recently gained attention as they are recurrently mutated in various cancer types and affect the very first step in the splicing cycle: the U1 snRNA, which is essential for 5′ splice site recognition, the U2 snRNP component SF3B1 and the U2 auxiliar factor subunit U2AF1, both of which are involved in 3′ splice site selection [15,16,17,18,19,20].