RNA enzymes have been developed that undergo self-sustained replication at a constant temperature in the absence of proteins. These RNA molecules amplify exponentially through a cross-replicative process, whereby two enzymes catalyze each other's synthesis by joining component oligonucleotides. Other RNA enzymes have been made to operate in a ligand-dependent manner by combining a catalytic domain with a ligand-binding domain (aptamer) to produce an 'aptazyme'. The principle of ligand-dependent RNA catalysis has now been extended to the cross-replicating RNA enzymes so that exponential amplification occurs in the presence, but not the absence, of the cognate ligand. The exponential growth rate of the RNA depends on the concentration of the ligand, allowing one to determine the concentration of ligand in a sample. This process is analogous to quantitative PCR (qPCR) but can be generalized to a wide variety of targets, including proteins and small molecules that are relevant to medical diagnostics and environmental monitoring.