Germline mutations in DICER1 underlie a rare syndrome associated with susceptibility to pleuropulmonary blastoma, cystic nephroma and ovarian sex-cord stromal tumors. David Huntsman, Gregg Morin and colleagues (N. Engl. J. Med., published online 21 December 2011; doi:10.1056/NEJMoa1102903) now show that somatic mutations in DICER1 occur at high frequency in non–epithelial ovarian cancers. The authors performed transcriptome and exome sequencing in two Sertoli-Leydig cell tumors, four juvenile granulosa cell tumors and eight primitive germ cell tumors and identified four missense mutations in DICER1 affecting the metal-binding region of the RNase IIIb domain. They subsequently sequenced the portion of DICER1 encoding this region in 101 additional non-epithelial ovarian tumors and identified somatic mutations in 60% of Sertoli-Leydig cell tumors. They also found similar mutations in 1 of 14 nonseminomatous testicular germ cell tumors and in 2 of 5 embryonal rhabdomyosarcomas. Functional studies showed that these mutations impair RNase IIIb activity but do not affect the activity of RNase IIIa. On the basis of these findings, the authors propose that these mutations result in an oncogenic miRNA profile whose pathogenicity may be restricted to specific cell types or developmental settings, thereby accounting for the high prevalence of these mutations in particular tumor types.