Circular non-coding RNA ANRIL modulates ribosomal RNA maturation and atherosclerosis in humans. Circular RNAs (circRNAs) are broadly expressed in eukaryotic cells, but their molecular mechanism in human disease remains unclear. This study shows that circular antisense non-coding RNA in the INK4 locus (circANRIL), which is transcribed at a locus of atherosclerotic cardiovascular disease on chromosome 9p21, confers atheroprotection by controlling ribosomal RNA (rRNA) maturation and modulating pathways of atherogenesis. CircANRIL binds to pescadillo homologue 1 (PES1), an essential 60S-preribosomal assembly factor, thereby impairing exonuclease-mediated pre-rRNA processing and ribosome biogenesis in vascular smooth muscle cells and macrophages. As a consequence, circANRIL induces nucleolar stress and p53 activation, resulting in the induction of apoptosis and inhibition of proliferation, which are key cell functions in atherosclerosis. These findings identify circANRIL as a prototype of a circRNA regulating ribosome biogenesis and conferring atheroprotection, thereby showing that circularization of long non-coding RNAs may alter RNA function and protect from human disease. Deep sequencing combined with novel bioinformatics approaches led to the discovery that a significant portion of the human transcriptome is spliced into RNA loops. These circular RNAs (circRNAs) do not retain the exon order defined by their genomic sequence and are thought to originate from non-canonical splicing of a 5' splice site to an upstream 3' splice site. Recent studies suggest that exon circularization may depend, in part, on inverted repeats or flanking intronic complementary sequences, but little is known about the functions of these highly stable RNA forms. Before the finding that circRNAs are abundantly transcribed in humans, there were few reports of circRNAs in mammals. One of the earliest examples is the sex determining region of Chr Y (Sry) gene in mice, the Y chromosome encoded master regulator of the testis-determining pathway. Sry may be expressed as circular and linear transcripts and circularization is thought to be a mechanism to escape translation. Sry was also shown to serve as a competing endogenous RNA of miRNA-138, and a similar 'miRNA sponging' function has been demonstrated for a transcript antisense to cerebellar degeneration related protein 1 (CDR1as/ciRS-7). CDR1as contains approximately 70 binding sites for miR-7 and acts to suppress miR-7 activity, resulting in increased levels of miR-7 target genes and functions. However, only few circRNAs harbour multiple binding sites for miRNAs, suggesting that these abundant RNAs may have other unknown regulatoryCircular non-coding RNA ANRIL modulates ribosomal RNA maturation and atherosclerosis in humans. Circular RNAs (circRNAs) are broadly expressed in eukaryotic cells, but their molecular mechanism in human disease remains unclear. This study shows that circular antisense non-coding RNA in the INK4 locus (circANRIL), which is transcribed at a locus of atherosclerotic cardiovascular disease on chromosome 9p21, confers atheroprotection by controlling ribosomal RNA (rRNA) maturation and modulating pathways of atherogenesis. CircANRIL binds to pescadillo homologue 1 (PES1), an essential 60S-preribosomal assembly factor, thereby impairing exonuclease-mediated pre-rRNA processing and ribosome biogenesis in vascular smooth muscle cells and macrophages. As a consequence, circANRIL induces nucleolar stress and p53 activation, resulting in the induction of apoptosis and inhibition of proliferation, which are key cell functions in atherosclerosis. These findings identify circANRIL as a prototype of a circRNA regulating ribosome biogenesis and conferring atheroprotection, thereby showing that circularization of long non-coding RNAs may alter RNA function and protect from human disease. Deep sequencing combined with novel bioinformatics approaches led to the discovery that a significant portion of the human transcriptome is spliced into RNA loops. These circular RNAs (circRNAs) do not retain the exon order defined by their genomic sequence and are thought to originate from non-canonical splicing of a 5' splice site to an upstream 3' splice site. Recent studies suggest that exon circularization may depend, in part, on inverted repeats or flanking intronic complementary sequences, but little is known about the functions of these highly stable RNA forms. Before the finding that circRNAs are abundantly transcribed in humans, there were few reports of circRNAs in mammals. One of the earliest examples is the sex determining region of Chr Y (Sry) gene in mice, the Y chromosome encoded master regulator of the testis-determining pathway. Sry may be expressed as circular and linear transcripts and circularization is thought to be a mechanism to escape translation. Sry was also shown to serve as a competing endogenous RNA of miRNA-138, and a similar 'miRNA sponging' function has been demonstrated for a transcript antisense to cerebellar degeneration related protein 1 (CDR1as/ciRS-7). CDR1as contains approximately 70 binding sites for miR-7 and acts to suppress miR-7 activity, resulting in increased levels of miR-7 target genes and functions. However, only few circRNAs harbour multiple binding sites for miRNAs, suggesting that these abundant RNAs may have other unknown regulatory