Standard & Poor's said Wednesday that flash memory company

SanDisk

(SNDK)

will replace

Chiron

( CHIR) in the

S&P 500

index.

Chiron is set to be acquired by pharmaceutical giant

Novartis

(NVS) - Get Report

in a deal that is pending final approval. S&P said the index change will go into effect after the close of trading April 19.

Scientific Games

(SGMS) - Get Report

, a provider of gaming and lottery products, will replace SanDisk in the S&P MidCap 400.

Shares of SanDisk jumped $4.49, or 7.6%, to $63.96 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. Shares of Scientific Games also got a boost, adding $1.68, or 4.8%, to $36.80.

On Tuesday, Standard & Poor's said

Legg Mason

(LM) - Get Report

will be added to the S&P 500 to replace

Guidant

( GDT), which is being bought by

Boston Scientific

(BSX) - Get Report

.