S&P Dow Jones Indices announced late Friday changes to various S&P indices, coinciding with the quarterly rebalance. Notable among the additions to the prestigious $S&P 500 Index (.SPX.US)$ was cybersecurity company $CrowdStrike (CRWD.US)$.

What Happened: CrowdStrike will replace $Robert Half (RHI.US)$ in the S&P 500 Index, while investment management company $KKR & Co (KKR.US)$ and $GoDaddy (GDDY.US)$ will take the place of $Comerica (CMA.US)$ and $Illumina (ILMN.US)$, respectively in the broader gauge.

The changes will take effect, prior to the open of trading on Monday. S&P Dow Jones said the changes ensure each index is more representative of its market capitalization range.

Illumina will be relegated to the S&P MidCap 400 Index in lieu of GoDaddy, while Robert Half and Comerica will replace $Anywhere Real Estate (HOUS.US)$ and $Adtran (ADTN.US)$ in the S&P SmallCap 600 Index. Gene sequencing company Illumina has announced plans to spin off cancer blood test making unit Grail on June 24.

S&P Dow Jones also announced a host of changes to its S&P MidCap 400 and the S&P 600 SmallCap indices.

Notable among the companies that have been kept out of the S&P 500 list are AI-powered data analytics company $Palantir (PLTR.US)$ and computer and peripherals maker $Dell Technologies (DELL.US)$. Incidentally, Palantir investors had anticipated inclusion in the S&P 500 at the previous quarterly rebalance in early March.

Why It's Important: The S&P 500 Index is considered the best single gauge of large-cap U.S. equities and includes 500 leading companies, covering approximately 80% of available market capitalization. The S&P 500, a market-cap weighted index, is rebalanced quarterly, usually on the third Friday of March, June, September and December.

During the rebalancing process, companies are typically removed or added based on the most recent share count and float of constituent companies. Their weights in the index are adjusted accordingly.

The eligibility criteria for S&P 500 Index inclusion are market capitalization of $15.8 billion or more, an annual dollar value traded to float-adjusted market capitalization greater than 0.75, a listing in either NYSE or NASDAQ, and a primary listing in the U.S. There is a a trading-volume criterion as well.

Index inclusion is typically positive for stocks, as funds holding portfolios mirroring the index will buy them to maintain the respective weightings of the component stocks. The stock will also benefit from increased volume and liquidity due to purchases by index funds and ETFs.

Following the announcement, CrowdStrike rose 5.35% to $367.80 in after-hours trading on Friday, according to Benzinga Pro data. KKR & Co. climbed 7.72% and GoDaddy added 3.72%.