In this study, we developed a novel application of the highly sensitive immuno-polymerase chain reaction (PCR) assay method (designated as MUSTag for Multiple Simultaneous Tag) for measuring GLA protein levels in clinical samples and determined the GLA concentrations in serum and plasma from male patients with classic Fabry disease, female patients with Fabry disease, male patients harboring p.E66Q, and control subjects. This evidence concerns the gene GLA and Fabry disease.