This family includes small ribosomal subunit S9 from prokaryotes and S16 from metazoans. This domain is predicted to bind to ribosomal RNA [1]. This domain is composed of four helices in the known structure. However the domain is discontinuous in sequence and the alignment for this family contains only the first three helices.

1: The crystal structure of ribosomal protein S4 reveals a two-domain molecule with an extensive RNA-binding surface: one domain shows structural homology to the ETS DNA-binding motif. Davies C, Gerstner RB, Draper DE, Ramakrishnan V, White SW; EMBO J 1998;17:4545-4558. PMID:9707415