Citigroup has closed on its purchase of S.C.-based Automated Trading Desk in a deal valued at $637 million in cash and stock.

The value of the takeover actually dropped from $680 million when it was announced in July due to a drop in Citigroup’s stock price.

ATD, which was privately held, closed on $60 million earlier this year from Technology Crossover Ventures.

The company, which was launched 18 years ago, provides automated stock-trading technology. It operates from a 16-acre campus near Charleston, S.C.