Example. If, with respect to stock transferred pursuant to the timely exercise of a statutory option, there is a distribution of new stock to which section 305(a) is applicable, and if there is a disposition of such new stock before the expiration of the applicable holding period required with respect to the stock originally acquired pursuant to the exercise of such option, such disposition makes section 421 inapplicable to the transfer of the original stock pursuant to the exercise of the option to the extent that the disposition effects a reduction of the individual's total interest in the old and new stock. However, if the new stock, as well as the old stock, is not disposed of before the expiration of the holding period required with respect to the original stock acquired pursuant to the exercise of the option, the special tax treatment provided by section 421 is applicable to both the original shares and the shares acquired by virtue of the distribution to which section 305(a) applies.
26 C.F.R. §1.424-1