NOB1P is a 412 amino acid nuclear protein that is involved in proteosome biogenesis and is required for the final step in 18S rRNA maturation. NOB1P contains a PIN domain, which functions as a nuclease in nonsense-mediated mRNA decay and is required for pre-rRNA cleavage. NOB1P interacts with Rent2, which is involved in nonsense-mediated decay of mRNAs containing premature stop codons. Expressed in placenta, spleen, endothelial cells, liver and lung, NOB1P is essential for the synthesis of 40S ribosome subunits. Supression of the gene encoding NOB1P inhibits the processing of the 20S pre-rRNA to the mature 18S rRNA, therefore leading to accumulation of high levels of 20S pre-rRNA with degradation intermediates.