This study reports an experimental system that allows the visualization of transcription and mRNA splicing in living human cells, with single-molecule resolution. Using this approach, the authors obtained key insights into splicing dynamics. First, they inserted GFP-tagged versions of the coat protein of bacteriophage MS2 or the antiterminator protein N of bacteriophage λ into each of the two introns of the β-globin gene and followed splicing dynamics using microscopy. They observed that β-globin intron excision occurs in 20–30 seconds, with the first intron having a shorter lifetime than the second (terminal) intron, which suggests that it is excised while the terminal intron is still present. Moreover, using a mouse immunoglobulin M (IgM) gene engineered to contain one intron between exons M1 and M2, the authors showed that splice site strength also influences splicing kinetics, as introns with a strong polypyrimidine tract (Py tract; an essential splicing signal) had a shorter lifetime than those with a weak Py tract.