Nature Reviews Molecular Cell Biology (2018) https://doi.org/10.1038/s41580-018-0059-1 Published online 18 September 2018

In Figure 1b, the GHG sequence motif in the primary microRNA has been moved to the basal stem and the ruler of the basal stem has been shortened to more precisely delineate 11 base pairs. The changes have been made in the HTML and PDF versions of the manuscript.