Jane Street

Jane Street writes its trading systems in OCaml, a functional language almost no other firm uses, picked for correctness over familiarity. That contrarian engineering choice sits under one of the world's largest market makers.

Four ex-Susquehanna traders, Tim Reynolds, Rob Granieri, Marc Gerstein, and Michael Jenkins, started it in 1999 trading options and ADRs on the old Amex floor, then built the ETF franchise that made the firm. It is one of the authorized participants that create ETF shares directly with issuers, and it has since pushed into equities, fixed income, and FX across hundreds of venues worldwide. All of it trades firm capital. Jane Street stays resolutely private, with Granieri the last founder still in place.