IRC § 1361(b)(3)
Termination of S and QSub Elections Due to 80% Acquisition of S Corp by C Corp
When the common parent of a consolidated group acquires 80 percent of an S corporation, the S election terminates and the subsidiary’s QSub election terminates with it as both join the buyer’s consolidated group.
Treas. Reg. § 1.1361-5(a)(4), Example 4 • Qualified Subchapter S Subsidiaries