Abstract
Thirty-five years ago, as young graduate students, we had the pleasure and privilege of being in Joan Steitz's laboratory at a pivotal point in the history of RNA molecular biology. Introns had recently been discovered in the laboratories of Philip Sharp and Richard Roberts, but the machinery for removing them from mRNA precursors was entirely unknown. This Retrospective describes our hypothesis that recently discovered snRNPs functioned in pre-mRNA splicing. The proposal was proven correct, as has Joan's intuition that small RNAs provide specificity to RNA processing reactions through base pairing in diverse settings. However, research over the intervening years has revealed that both splice site selection and splicing itself are much more complex and dynamic than we imagined.