CBS and retinitis: Additionally, Si et al. (2013) reported that DR was associated with significant decreases in retinal H2S levels and expression of the H2S synthesizing enzymes CSE, CBS, and 3MST (i.e., DR resulted in retinal H2S deficiency) and also with increased mitochondrial permeability and respiration (Si et al., 2013), strongly suggesting extensive mitochondrial dysfunction and H2S deficiency in the retina in DR.