Ultragenyx Gene Therapy is focused on advancing AAV-based gene therapies for rare, but serious, genetic diseases. Potency assessment for such advanced therapeutic product is of critical importance, but can also be very challenging. Because a reliable potency assay is required as early as Phase I/II of product development, we have developed a relative potency assay by measuring the expression of transgene specific mRNA after the transductions of appropriate target cells. Assay was miniaturized and multiplexing was implemented to improve assay throughput and performance. This potency assay shows a high level of robustness and throughput, and requires only minor adjustment while modifying it for novel products. This presentation describes how this platform potency assay was developed and successfully adapted for two different gene therapy products.