Enables transcription corepressor activity. Acts upstream of or within inner cell mass cell proliferation; negative regulation of transcription, DNA-templated; and skeletal muscle cell differentiation. Located in nucleus. Part of COP9 signalosome. Is expressed in several structures, including alimentary system; genitourinary system; integumental system; nervous system; and sensory organ. Orthologous to human COPS2 (COP9 signalosome subunit 2). [provided by Alliance of Genome Resources, Apr 2022]