Abstract: Disclosed are a protein marker, indicative of an increase in differentiation into adipocytes, comprising an XBP1(S) having an amino acid sequence of SEQ ID NO. 1, 2 or 3, and the uses thereof in developing a promoter of adipocyte differentiation and a method for promoting adipocyte differentiation, a repressor of adipocyte differentiation and a method for repressing adipocyte differentiation, an agent and a method for screening a repressor of adipocyte differentiation, and a method for reducing rosiglitazone's side effect of causing obesity. Also, provided is a protein marker, indicative of an increase in differentiation into adipocytes, comprising an XBP1(U) having an amino acid sequence of SEQ ID NO. 4, 5 or 6. When targeting the XBP1(S) gene or protein, a compound capable of blocking or restraining differentiation into adipocytes can be used to develop an agent for the prevention and treatment of obesity.