Single domain aspartyl proteases from retroviruses, retrotransposons, and badnaviruses (plant dsDNA viruses). These proteases are generally part of a larger polyprotein; usually pol, more rarely gag. Retroviral proteases appear to be homologous to a single domain of the two-domain eukaryotic aspartyl proteases such as pepsins, cathepsins, and renins (Pfam:PF00026).