Figure 1: H2S biosynthesis, functions, and metabolism. So let us first look at how H2S is endogenously produced. In mammalian cells, H2S can be endogenously produced through the transsulfuration pathway. With cysteine as the main substrate, CBS or CSE, which we called cystathionine gamma-lyase or cystathionine beta-synthase, can catalyze cysteine into H2S and other products. The expression of CBS and CSE is tissue-specific, CBS is mostly expressed in brain, and CSE is in the cardiovascular system and other big tissues, such as liver and kidney. The half-life of H2S inside the cells is very fast; it is estimated in several seconds and can be quickly oxidized, scavenged, or broken down by different ways. Now it has been widely recognized that H2S plays very important physiological role in the whole body and also cells; for example, it can relax blood vessel and acts as an endothelial deprived hyperpolarizing factor, can enhance long term potentiation and help memory, and can control energy generation and regulate metabolism, inflammation; more importantly, H2S is required for cell fate decision; that is the topic we are going to talk about in the following.