Evidence on the co-transcriptionality of splicing and on a role for the transcription machinery on splice site selection started to be published before the launching of RNA. However, it was during the last 20 years that initial suspicion and speculation gave room to a profuse body of evidence supporting a radical change of view on the regulatory mechanisms of splicing, originally conceived as a purely post-transcriptional event. Surprisingly, the first evidence for co-transcriptional splicing is still one of the strongest ones, probably because “seeing is believing”: In 1988 Beyer and Osheim performed cytological examination of Drosophila genes caught in active transcription and produced beautiful and compelling EM images revealing that many introns are excised as the mRNA is being synthesized, before RNA polymerase II (RNAPII) reaches the end of the gene. In agreement with this observation, a few years later, using light microscopy, Jeanne Lawrence confirmed that pre-mRNAs are constrained from free diffusion and that splicing appeared to occur within small tracks in the vicinity of gene loci. This idea would be reinforced by similar but not identical observations from David Spector's lab showing that splicing factors are recruited from speckles to the sites of transcription upon transcriptional activation. However, back to the Beyer and Osheim observation, if splicing was co-transcriptional, could it also be mechanistically coupled to transcription? In April 1993, Arno Greenleaf published an “Open Question” article in TiBS with a speculative model in which positively charged splicing factors could be tethered to the phosphorylated, and therefore negatively charged, carboxy-terminal domain (CTD) of elongating RNAPII, which would then help splicing factors to gain access to their binding sites in the nascent pre-mRNA. This provocative idea proved to be extremely fruitful to investigate CTD functions other than transcription and opened the whole field of coupling. Indeed, in the following years, a physical association of the highly phosphorylated form of RNAPII large subunit with splicing factors and spliceosome components as well as a role in stimulating splicing was demonstrated by the Corden, Berezney, and Manley labs. These in vitro experiments were paralleled by an in vivo proof provided by a seminal Nature paper of 1997 from the Bentley lab showing that when transcription in cultured cells is performed by an RNAPII bearing a truncated CTD, RNA splicing, 3′ end processing, and transcriptional termination are affected. Bentley proposed then the concept of “mRNA factory,” suggesting that RNAPII and its associated factors not only perform transcription but are also key for the co-transcriptional covalent modifications of the pre-mRNA. The CTD being a particular feature of RNAPII, these findings were highly consistent with observations of the mid-'80s that RNA processing was less efficient when intron-bearing genes were put under the control of RNAPI or RNAPIII promoters, which would be later complemented by the Rosbash lab extending similar observations to T7 phage promoters. At present there is some controversy as to whether splicing factors interact directly with RNAPII. Using reporter genes embedded in bacterial artificial chromosomes to guarantee chromatin environments of the stably transfected constructs similar to those of the corresponding endogenous genes, Karla Neugebauer showed in 2009 that all SR protein interactions with RNAPII are RNase-sensitive, suggesting that SR proteins are not preassembled with the enzyme but recruited to nascent mRNA.