26 USC 860B - Taxation of holders of regular interests

(a) General rule 
In determining the tax under this chapter of any holder of a regular interest in a REMIC, such interest (if not otherwise a debt instrument) shall be treated as a debt instrument.
(b) Holders must use accrual method 
The amounts includible in gross income with respect to any regular interest in a REMIC shall be determined under the accrual method of accounting.
(c) Portion of gain treated as ordinary income 
Gain on the disposition of a regular interest shall be treated as ordinary income to the extent such gain does not exceed the excess (if any) of
(1) the amount which would have been includible in the gross income of the taxpayer with respect to such interest if the yield on such interest were 110 percent of the applicable Federal rate (as defined in section 1274 (d) without regard to paragraph (2) thereof) as of the beginning of the taxpayers holding period, over
(2) the amount actually includible in gross income with respect to such interest by the taxpayer.
(d) Cross reference 
For special rules in determining inclusion of original issue discount on regular interests, see section 1272 (a)(6).