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Mammals span an astonishing array of diverse body forms and appearances. In this issue, Chen et al. describe a framework for modeling the evolution of gene expression across mammals to quantify how expression drives phenotypic variation. Understanding the evolutionary histories of gene expression pathways offers important insight into healthy and disease levels of gene expression in humans. (Cover artwork by Anna Hupalowska, www.annahupalowska.com. [For details, see Chen et al., pp. 53–63.])
