Word that Standard & Poor's will add Celgene ( CELG) to its S&P 500 index was lifting the biotech company's stock Tuesday. Lately, Celgene was up 6% to $52.80. Shares of companies added to the S&P 500 often rise because fund managers who track the index have to own the stock. Celgene is set to replace AmSouth Bancorp ( ASO), which is being acquired by Regions Financial ( RF) in a deal expected to close by Nov. 6. Regions, in turn, will supplant RadioShack ( RSH) in the S&P 100, Standard & Poor's said Monday. RadioShack, with a market capitalization of about $2.4 billion, ranked 100th in the S&P 100 as of the previous close of trading. The moves will occur after the close of trading Nov. 3.