Also known as MRT4_HUMAN, MRTO4, C1orf33, MRT4. Component of the ribosome assembly machinery. Nuclear paralog of the ribosomal protein P0, it binds pre-60S subunits at an early stage of assembly in the nucleolus, and is replaced by P0 in cytoplasmic pre-60S subunits and mature 80S ribosomes. Associates with the pre-60S ribosomal particle.