A blood-based ELP-1 protein assay may provide a reliable way to monitor gene target engagement.<h4>Design and methods</h4>Using a newly developed radioimmunoassay, we quantified ELP1 protein levels in peripheral blood samples collected from 59 homozygous FD patients carrying the IVS20 + 6T>C mutation and 66 heterozygous carriers. The gene discussed is ELP1; the disease is Fabry disease.