Standard & Poor's will add Celgene ( CELG) to its S&P 500 index to replace AmSouth Bancorp ( ASO).

Regions Financial ( RF) is acquiring AmSouth 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 close of trading Monday.

The moves will occur after the close of trading Nov. 3.

Shares of Celgene, a biotech company, climbed $2.15, or 4.3%, to $52 in after-hours trading. Shares of companies added to the S&P 500 often rise because fund managers who track the index have to own the shares.