Figure 1.

De novo creation of genes by overprinting. Typically, genomic regions contain a single gene encoding a protein product in one of the 6 reading frames (for example, reading frame +1 in panel (A). The same genomic region does not encode any protein in the other 5 reading frames, either because of absence of start codons (as in reading frame +3 and −3, marked with ×) or because of the presence of stops shortly after a start codon (as in reading frames +2, −1 and −2, marked with ●). The occurrence of single point mutations in one of these 5 reading frames can give rise to a new protein-coding gene, either by creating a new start codon (as in reading frame +3 in panel (B) or by eliminating a stop codon (as in reading frame −2 in panel (C). When this occurs, the original gene is called ‘ancestral’ or ‘overprinted’, while the new one is called ‘novel’ or ‘overprinting’. In rare cases, both events can occur over time (as in the case of HTLV-1 and HIV-1), thus giving rise to genomic regions that contain three overlapping protein-coding genes.