This document provides specific guidance for selecting genes to include on Xenium add-on panels. The Xenium Custom Panel design process uses single cell expression data to calculate cell type-specific expression profiles, which represent a model of which genes are co-expressed. The Xenium Panel Designer provides a curated selection of single cell references to use. If none of the references match the tissue or condition being assayed, 10x Genomics recommends providing single cell data that matches as closely as possible. Once genes are selected, customers will work with 10x Genomics to finalize details of the panel.