Shortly after the discovery that mRNA primary transcripts contain long, non-coding intervening sequences (introns) by Sharp and Roberts, Joan Steitz began work to characterize the biochemical mechanism of intron excision. The curious observation that a sequence found in the 5´ region of the U1 snRNA exhibited extensive base pairing complementarity with conserved sequences across 5´ splice junctions in hnRNA transcripts prompted speculation that certain snRNAs may be involved in recognizing splice site boundaries through RNA:RNA contacts. Only recently have atomic crystal structures revealed demonstrably that the original conjecture was indeed correct, even if the complexity of these interactions were not fully realized at the time.