An unknown endonuclease in the nucleolus cleaves at site E (site 2a in mouse) of 21S pre-rRNA, yielding 18SE pre-rRNA (Preti et al. 2013, Sloan et al. 2013). Evidence also indicates that 18SE may also be produced by an exonucleolytic pathway (Carron et al. 2011, Sloan et al. 2013). BYSL (Bystin, ENG1), SKIV2L2 (MTR4), and the exonuclease activity of EXOSC10 (RRP6), all associated with the exosome, are required for formation of 18SE by the exonucleolytic pathway (Sloan et al. 2013). Sequencing indicates that 18SE molecules can have variable ends, presumably due to exonuclease activity (Preti et al. 2013).