Ribosomes, the organelles that catalyze protein synthesis, are composed of a small subunit (40S) and a large subunit (60S) that consist of over 80 distinct ribosomal proteins. Mammalian ribosomal proteins are encoded by multigene families that contain processed pseudogenes and one functional intron-containing gene within their coding regions. RPS4Y2 (ribosomal protein S4, Y-linked 2), RPS4Y1 (ribosomal protein S4, Y-linked 1) and RPS4X (ribosomal protein S4, X-linked) are proteins that contain one S4 RNA-binding domain and exist as components of the 40S ribosomal subunit, indicating a possible role in protein synthesis.