Enables HLH domain binding activity. Involved in negative regulation of transcription by RNA polymerase II. Acts upstream of or within positive regulation of I-kappaB kinase/NF-kappaB signaling. Located in nuclear body. Is expressed in several structures, including early embryo; forebrain; forelimb bud; limb; and oocyte. Orthologous to human UBE2I (ubiquitin conjugating enzyme E2 I). [provided by Alliance of Genome Resources, Apr 2022]