An unknown endonuclease cleaves site 1 (also called site A1), removing the remainder of the 5' external transcribed spacer (5' ETS) from 30S pre-rRNA containing the 18S rRNA (Freed et al. 2012, Tomecki et al. 2015). Sixteen proteins of the small ribosomal subunit are required for processing sequences flanking the 18S rRNA in the 30S pre-rRNA (O'Donohue et al. 2010) presumably due to their assembly onto pre-rRNA as processing proceeds. Additionally, FCF1 (hUTP24, part of the SSU processome) is required for cleavage at site 1 (Tomecki et al. 2015) and NOL11 and CIRH1A (Cirhin, part of the UTPA complex) interact and are required for cleavage at site 1 and other sites (A', A0, and 2) (Freed et al. 2012). FTSJ3 and NIP7 interact and are required for processing at sites 1, 2, and A0 (Morello et al. 2011).