. gearon 2019s of his right to require the company to purchase his interest in atc south america , all options granted pursuant to the atc south america stock option plan vested in full and were exercised . upon exercise of these options , the holders received 4428 shares of atc south america ( representing a 7.8% ( 7.8 % ) interest ) , net of 1596 shares retained by the company to satisfy employee tax withholding obligations . ( see note 11