T in vivo the Roc1b RING domain cannot productively interact (i.e. stimulate ubiquitylation of critical targets) with all of the E2s that Roc1a does. This may occur at the level of direct binding, such that there are some E2s that only bind the RING domain of Roc1a and not Roc1b, or at the level of stimulating ubiquitin transfer to substrate in the context of a fully assembled CDL. A similar obser