Required for processing of 20S pre-rRNA precursor and biogenesis of 40S ribosomal subunits. May be required for trophinin-dependent regulation of cell adhesion during implantation of human embryos. Binds trophinin, tastin and cytokeratins. Associated with 40S ribosomal subunits. Found in the placenta from the sixth week of pregnancy. Was localized in the cytoplasm of the syncytiotrophoblast in the chorionic villi and in endometrial decidual cells at the uteroplacental interface. After week 10, the level decreased and then disappeared from placental villi. HeLa cells lacking BYSL show a delay in the processing of the 18S rRNA component of the 40S ribosomal subunit. HT-H cells lacking BYSL show trophinin-independent signaling through ERBB4. Belongs to the bystin family. Truncated N-terminus.