Gap Inc. /zigman2/quotes/206554267/composite GPS -3.64% is leaving the S&P 500 index /zigman2/quotes/210599714/realtime SPX -0.81% , index provider S&P Dow Jones Indices said late Wednesday, underscoring the challenges faced by retailers in recent years. Constellation Energy Corp. /zigman2/quotes/232373307/composite CEG -3.26% will replace Gap in the key equity index, with the retailer, parent to Banana Republic, Old Navy and Athleta in addition to the namesake store brand and others, replacing fast-food chain Jack in the Box Inc. /zigman2/quotes/206571438/composite JACK -2.56% in the S&P MidCap 400. Jack in the Box will replace Spectrum Pharmaceuticals Inc. /zigman2/quotes/205199409/composite SPPI -2.59% in the S&P SmallCap 600, S&P Dow Jones said. The changes take effect on Feb. 3. Exelon Corp. /zigman2/quotes/205982254/composite EXC -0.62% is spinning off Constellation Energy in a transaction expected to close Feb. 2.