library_construction_protocol: Mouse embryonic stem cells were harvested and Drosophil cells were added in a ratio of 1:3 (drosophila:mouse). Total RNA was extracted with the RNeasy Mini kit (Qiagen 74104). Poly A mRNA was isolated with NEBNext Poly(A) mRNA magnetic isolation module (E7490). NEBNext Ultra II Directional RNA Library Prep Kit for Illumina (E7760)