Required for processing of 20S pre-rRNA precursor and biogenesis of 40S ribosomal subunits. Binds trophinin, tastin and cytokeratins. Associated with 40S ribosomal subunits. High levels in preimplantation embryos, bone marrow, brain, testis and ovary. High levels of maternal transcript in unfertilized eggs. High levels up to blastocyst stage. Mice show embryonic lethality around stage 6.5 dpc shortly after implantation. Mice lacking maternal Bysl transcript upon injection of siRNA into fertilized eggs are arrested at the 16-cell stage, fail to induce trophectoderm, and show altered morphology of developing nucleoli. Embryonic stem cells lacking Bysl show accumulation of pre-20S rRNA precursors and a reduction of 40S ribosomal subunits in the cytoplasm. Belongs to the bystin family.