 The larger size resulting from more ribosomes and protein is not associated with increased proliferation, however, and this ARF-null phenotype occurs even though the normal basal levels of Arf are usually low.Knocking down ARF with siRNA to exon 1β results in increased rRNA transcripts, rRNA processing, and ribosome nuclear export.The unrestrained ribosome biogenesis seen when NPM is not bound to ARF does not occur if NPM is also absent.