Ubiquitin-conjugating enzyme E2 (UEV1) was initially discovered as a protein similar in sequence and structure to the E2 ubiquitin-conjugating enzymes but lacking their enzymatic activity. There are at least two variants and multiple isoforms of UEV1. In particular, UEV1A (Ubiquitin-conjugating enzyme E2 variant 1 isoform A) has recently been shown to be an important component of the Toll-like receptor and IL-1R signaling pathway. Signals from these pathways are relayed by a number of downstream molecules such as MyD88 and tumor necrosis factor receptor associated factor (TRAF6), ultimately activating various kinases and transcription factors. UEV1A is part of a dimeric ubiquitin-conjugating enzyme complex also containing Ubc13 (ubiquitin-conjugating enzyme 13) that together with TRAF6 activates TAK1, a member of the mitogen-activated protein kinase kinase kinase family. The Ubc13-UEV1A complex also mediates the Lys-63 ubiquitination of TRAF-6, and this ubiquitination is essential for TAK1 activation.