Welcome to the S&P 500.

Welcome to the big show, Twitter. 

Twitter Inc. (TWTR) shares spiked in pre-market trading Tuesday after index provider S&P Dow Jones said the social media group would be added to the benchmark S&P 500 later this week.

Twitter will replace Monsanto Co. (MON) in the benchmark after the group's $63.5 billion merger with German chemicals group Bayer AG closes on Thursday June 7. S&P Dow Jones also said Netflix Inc. (NFLX) would replace Monsanto, which will be blended into Bayer and no longer operate as a separate company, in the S&P 100 when the deal is complete, the provider said in a statement late Monday. Twitter will also sit in the Internet Software & Services Sub-Industry index.

Wow Twitter and Netflix get added to the SandP 500. Index funds like to buy high. Great additions however. Congrats $TWTR $NFLX

— Ross Gerber (@GerberKawasaki) June 4, 2018

Twitter shares were marked 4.86% higher in pre-market trading Tuesday, indicating an opening bell price of $39.72 each, the highest since April 2015 and a move that would extend its year-to-date gain past 62% and value the social media franchise at just under $30 billion.

Tech stocks comprise the largest portion of the S&P 500's daily movement, with a 25.78% weighting on the benchmark, up from 23.78% at the end of last year. By some estimates, the weighting is the highest since the one-year peak achieved just prior to the dot-com collapse in late 2000. 

Over on RealMoney Pro, one of the TheStreet's technical experts, Timothy Collins, argues the shares could continue their extraordinary run deep into the second half of the year.  

"The microblogging service has found a foothold with buyers and I don't think it lets it go in 2018," he wrote late last month. The weekly chart showcases not only a strong trendline higher, but now a bullish consolidation pattern in the form of a cup and handle. Shares have pushed higher for roughly six months before spending the last two in a small pullback.

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