50-percent shareholder
(4) Special rules for application of subsection (A) Treatment of less than 5-percent shareholders Except as provided in subparagraphs (B)(i) and (C), in determining whether an ownership change has occurred, all stock owned by shareholders of a corporation who are not 5-percent shareholders of such corporation shall be treated as stock owned by 1 5-percent shareholder of such corporation. (B) Coordination with equity structure shifts For purposes of determining whether an equity structure shift (or subsequent transaction) is an ownership change— (i) Less than 5-percent shareholders Subparagraph (A) shall be applied separately with respect to each group of shareholders (immediately before such equity structure shift) of each corporation which was a party to the reorganization involved in such equity structure shift. (ii) Acquisitions of stock Unless a different proportion is established, acquisitions of stock after such equity structure shift shall be treated as being made proportionately from all shareholders immediately before such acquisition. (C) Coordination with other owner shifts Except as provided in regulations, rules similar to the rules of subparagraph (B) shall apply in determining whether there has been an owner shift involving a 5-percent shareholder and whether such shift (or subsequent transaction) results in an ownership change. (D) Treatment of worthless stock If any stock held by a 50-percent shareholder is treated by such shareholder as becoming worthless during any taxable year of such shareholder and such stock is held by such shareholder as of the close of such taxable year, for purposes of determining whether an ownership change occurs after the close of such taxable year, such shareholder— (i) shall be treated as having acquired such stock on the 1st day of his 1st succeeding taxable year, and (ii) shall not be treated as having owned such stock during any prior period. For purposes of the preceding sentence, the term “50-percent shareholder” means any person owning 50 percent or more of the stock of the corporation at any time during the 3-year period ending on the last day of the taxable year with respect to which the stock was so treated.