The Gam1 protein of the avian CELO adenovirus activates transcription through inhibition of histone deacetylase 1 (HDAC1). We investigated the effect of Gam1 on both transient and stable transgene expression in Chinese hamster ovary (CHO) cells, one of the most commonly used mammalian hosts for the large-scale production of recombinant proteins. Transient expression of Gam1 increased reporter protein levels up to 4-fold in suspension cultures of CHO DG44 cells co-transfected with a reporter gene and up to 20-fold in recombinant CHO DG44-derived cell lines. The highest levels of activation were observed when the transgene was under the control of the human cytomegalovirus (HCMV) immediate early promoter/enhancer. Increases in recombinant protein expression in the presence of Gam1 were not accompanied by an enhancement of cell growth or viability. We conclude that Gam1 may serve as a useful genetic tool for increasing recombinant protein expression in CHO DG44 cells.