C11orf98 encodes a microprotein with dual roles in ribosome biogenesis and sister chr11 cohesion. In the nucleolus and nucleoplasm, C11orf98 functions as a nuclear assembly factor for pre-60S ribosomal particles 1, participating in the hierarchical maturation of human ribosomal subunits. The protein interacts with nucleolar proteins nucleophosmin and nucleolin 2, supporting its role in ribosome assembly. Beyond ribosome biogenesis, C11orf98 encodes RSMC (28S rRNA/ribosome and Sororin micro-cofactor), a microprotein that regulates sister chr11 cohesion during S-phase 3. PARP1-mediated PARylation of RSMC strengthens its interaction with Sororin, a cohesin-associated protein essential for establishing sister chr11 cohesion concurrent with DNA replication. This PARylation enhances both chr11 recruitment and anti-WAPL activity of Sororin, working synergistically with SMC3 acetylation. Overexpression of RSMC rescues cohesion defects induced by PARP inhibitors like olaparib 3, suggesting C11orf98 represents an understudied microprotein with critical roles in fundamental cellular processes. These findings highlight C11orf98 as a multifunctional regulator integrating ribosomal maturation with genomic stability maintenance.