If a qualified opportunity fund (QOF) fails the 90% requirement, it is subject to a penalty for each month it fails to meet the requirement in an amount generally equal to the amount by which it misses satisfying the 90% requirement, multiplied by the underpayment rate for the month, unless either the QOF has reasonable cause for the failure, or the one-time Qualified Opportunity Zone Business (QOZB) cure is applicable.