We have described the development and validation of a new gene-expression diagnostic test called Pheo-type for classification of PPGL tumors based on the underlying genetic driver of the disease. Our assay confirms that the predominant gene-expression subtypes of PPGL are closely linked to tumor genotype and for the first time show that classification can be achieved using a relatively small gene set. Pheo-type is similar to other clinical assays developed on the Nanostring platform for subtyping of solid tumors (50–52). As with these other studies, we have shown that robust gene-expression analysis can be achieved from small amounts of FFPE tissue material generating data that are comparable to other gene-expression platforms. The assay is rapid, largely automated, and cost effective, enabling the generation of results in 2 days from RNA extraction to data analysis, facilitating its implementation in a routine diagnostic laboratory.