A Competition between Stimulators and Antagonists of Upf Complex Recruitment in Human NMD

(A) A unified model where NMD is determined by the balance between 3′ UTR–associated factors that stimulate (such as the EJC) or antagonize (such as cytoplasmic PABP) recruitment of the hUpf complex (shown as spheres labeled 1–3) to the terminating ribosome.

(B) Mechanisms by which mammalian mRNAs with long 3′ UTRs may evade NMD (see Discussion for details).