Splicing remains an incompletely understood process. Recent findings suggest 
that chromatin structure participates in its regulation. Here, we analyze the 
RNA from subcellular fractions obtained through RNA-seq in the cell line K562. 
We show that in the human genome, splicing occurs predominantly during 
transcription. We introduce the coSI measure, based on RNA-seq reads mapping to 
exon junctions and borders, to assess the degree of splicing completion around 
internal exons. We show that, as expected, splicing is almost fully completed in 
cytosolic polyA+ RNA. In chromatin-associated RNA (which includes the RNA that 
is being transcribed), for 5.6% of exons, the removal of the surrounding introns 
is fully completed, compared with 0.3% of exons for which no intron-removal has 
occurred. The remaining exons exist as a mixture of spliced and fewer unspliced 
molecules, with a median coSI of 0.75. Thus, most RNAs undergo splicing while 
being transcribed: "co-transcriptional splicing." Consistent with 
co-transcriptional spliceosome assembly and splicing, we have found significant 
enrichment of spliceosomal snRNAs in chromatin-associated RNA compared with 
other cellular RNA fractions and other nonspliceosomal snRNAs. CoSI scores 
decrease along the gene, pointing to a "first transcribed, first spliced" rule, 
yet more downstream exons carry other characteristics, favoring rapid, 
co-transcriptional intron removal. Exons with low coSI values, that is, in the 
process of being spliced, are enriched with chromatin marks, consistent with a 
role for chromatin in splicing during transcription. For alternative exons and 
long noncoding RNAs, splicing tends to occur later, and the latter might remain 
unspliced in some cases.