PPBs are composed of malignant mesenchymal cells, often associated with benign epithelium.4 The majority of children with PPB are found to have germline DICER1 mutations, although penetrance of these mutations is low.6 Relatively little is known about the potential mechanisms through which mutations in the DICER1 gene may lead to tumor development. The gene discussed is DICER1; the disease is pleuropulmonary blastoma.