May be an associated component of the ribosome rather than a core structural subunit. May act as a translation initiation factor. Has a role in regulation of cell proliferation in the hematopoietic organs and the imaginal disks of larva (By similarity). Component of the 40S small ribosomal subunit. Interacts with sta. Detected on cytosolic polysomes (By similarity). Detected in ribosomes that are associated with the rough endoplasmic reticulum (By similarity). Belongs to the eukaryotic ribosomal protein eS21 family.