Component of the large ribosomal subunit (PubMed:12962325, PubMed:23636399, PubMed:25901680, PubMed:25957688). The ribosome is a large ribonucleoprotein complex responsible for the synthesis of proteins in the cell (PubMed:12962325, PubMed:23636399, PubMed:25901680, PubMed:25957688). {ECO:0000269|PubMed:23636399, ECO:0000269|PubMed:25901680, ECO:0000269|PubMed:25957688, ECO:0000305|PubMed:12962325}. This is the function of RPL21 (ribosomal protein L21, Ensembl gene identifier ENSG00000122026).