Fig. 8. uORF control of reinitiation and leaky scanning. (A) Reinitiation. Scanning ribosomes load at the uORF start codon and initiate translation there; subsequently, they reinitiate translation downstream. (B) Leaky scanning. Ribosomes scan and load at either the upstream or downstream start codons. (C) Discriminating reinitiation from leaky scanning. Adding cycloheximide (CYH) at T0 will trap ribosomes at the positions where they first load on the mRNA; adding it during steady-state translation (T5–T15) will trap ribosomes where they are loaded following the primary initiation event and subsequent reinitiation events.