Model for the function of USB1 in U6 snRNA processing according to Hilcenko et al.¹  The U6 is transcribed by RNA polIII and terminates with UUUU and a 3′OH. The transcript is then extended with U residues by TUTase. A residues can also be incorporated. USB1 chews back the 3′ tail, leaving the RNA with 5 terminal U residues and a 3′ cyclic phosphate, which cannot be a substrate for polyA polymerase TRF4. When USB1 is defective in PN, TRF4 can use the 3′OH terminus as a substrate and oligoadenylate the RNA, which can then be degraded by the exosome. Illustration by Nieves Perdigones.