F- Box plot showing Pol II pausing duration in RPB1-52R and RPB1-25R cells for genes encoding major isoforms with constitutive first exon longer than 100 bp. p value<2.2e-16 (Mann-Whitney U test) Box limits are the first and third quartiles, the band inside the box is the median. The ends of the whiskers extend the box by 1.5 times the interquartile range. Two independent biological replicates were analyzed. 