U2AF1 and cancer: In a step toward addressing this question, we prepared heterodimers of U2AF2 bound to an extended U2AF1 construct containing the zinc knuckle regions, and used smFRET to characterize the influence of these U2AF1 zinc knuckles, the cancer-associated S34F mutation, and a variety of representative 3′ splice site RNAs on the U2AF2 inter-RRM dynamics.