RNA$-$seq libraries were constructed by NEBNext Poly$($A$)$ mRNA Magnetic Isolation Module

$($NEB E$7490)$ and NEBNext Ultra II Non$-$Directional RNA Second Strand Synthesis Module

$($NEB E$6111).$ mRNA was purified with poly$-$T magnetic beads and first and second strand

cDNA was synthesized. The resulted cDNA was purified by AMPure XP beads $(1$:$1)$ and eluted

in $50$ L nucleotide$-$free water. The subsequent procedures were the same as described in

ChIP$-$seq library construction, except that the sequencing depth was $20$ million reads per

library. RNA$-$seq libraries were sequenced by Illumina Nova$-$seq platform with pair$-$end reads

of $150$ bp$.$

what feature of mRNA did the authors take advantage of to enrich for mRNA