Lance Barber, Raegan Revord and Montana Jordan have joined the cast of the Chuck Lorre and Steve Molaro comedy, with Jim Parsons set to narrate.

It's official: CBS is moving forward with its Big Bang Theory prequel.

The network has handed out a straight-to-series order for the 2017-18 broadcast season for Young Sheldon, The Hollywood Reporter has learned.

The news comes as the comedy created by Chuck Lorre and Steve Molaro had been casting ahead of production on the pilot this month. This is the first time that the network and producers Warner Bros. Television have confirmed the project. An episode count has not yet been determined.

Young Sheldon recently cast its star, setting Big Little Lies actor Iain Armitage as the young version of Jim Parsons' Sheldon Cooper, with Zoe Perry taking on the role of his mother, Mary Cooper, who was originally played by the actress' mother, Laurie Metcalf.

In picking up the comedy to series, CBS has also released Young Sheldon's formal logline, filled out its cast and set the pilot director. The project will revolve around a 9-year-old Sheldon Cooper (Armitage) as he lives with his family in East Texas and attends high school. The concept was originally described as Malcolm in the Middle with a young Sheldon. Lance Barber, Raegan Revord and Montana Jordan have also joined the cast. Details on their roles are being kept under wraps, though sources tell THR that Revord will play a young version of Sheldon's twin sister Missy; Barber will play George, Sheldon and Missy's father and Mary's husband; and Jordan will play George Jr., Sheldon's older brother.

Young Sheldon hails from Big Bang Theory creator Lorre and showrunner Molaro. Fellow Big Bang Theory co-creator Bill Prady is not attached to the prequel series. Jon Favreau is set to direct and executive produce the pilot. Parsons will narrate the comedy as Adult Sheldon. Lorre, Molaro, Parsons and Todd Spiewak will exec produce the single-camera series (marking a change from Big Bang Theory, which is a multicamera comedy). A premiere date has not yet been determined.

During Big Bang's 10-season run, many references have been made to the childhood of Sheldon, who grew up a gifted youngster in Texas with a religious mother. A prequel series gives the writers a lot of material to work with given Sheldon's history, as well as a structure to follow. Big Bang, unlike most series on TV, does not plot out its arcs and seasons ahead of time — the writers take it episode by episode.

The Young Sheldon series pickup comes as the pricey contracts for the seven lead stars of Big Bang expire at the end of its current 10th season. A new two-year deal is said to be in the works, as new deals with the cast are being drafted.

Young Sheldon is one of multiple spinoffs underway this season. ABC is also prepping a 1990s-set spinoff of The Goldbergs as well as a college-focused offshoot of Black-ish, starring Yara Shahidi. In an era of dwindling mass-audience TV hits, spinoffs are seen as a proven way to keep bankable franchises going. CBS is also looking to The Good Wife for its All Access spinoff, The Good Fight, and NBC is poised to launch an offshoot of The Blacklist with The Blacklist: Redemption, which will join its deep roster of Dick Wolf's Chicago spinoffs.

Including Young Sheldon, Lorre will likely have three shows on CBS next season, with his critically praised comedy Mom also expected to earn a renewal. He also has Netflix comedy Disjointed, starring Kathy Bates, set as part of his massive overall deal with Warner Bros. Television.

Young Sheldon is the latest straight-to-series order for the 2017-18 broadcast season. It joins ABC's Marvel drama Inhumans, produced in partnership with Imax; ABC's Ten Days in the Valley; Fox's Seth MacFarlane dramedy Orville; and NBC's Will and Grace revival.

Keep track of the latest news and castings at THR.com/PilotSeason and bookmark THR's handy guide.