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). Expressed both maternally and zygotically. Zygotic expression is high in middle-stage embryos, lower in middle-stage larvae, dropping through pupal development to be much lower in adults. Belongs to the eukaryotic ribosomal protein eS21 family.