DICER1 gene is located on chromosome 14q32.13 and plays a crucial role in the control of protein translation; its product, dicer protein, is a ribonuclease (RNase) III endoribonuclease which is essential for the production of microRNAs (miRNA) which are formed by the cleavage of pre-miRNA or double-stranded RNA1–4. RNase III contains two domains, IIIa and IIIb which cleave 3p miRNA and 5p miRNA from the 3′ and 5′ pre-miRNA, respectively. These cleavages require magnesium ions at the interface between the IIIa and IIIb domains and the miRNA; this magnesium dependent catalytic processing occurs at specific residues, E1320, E1564, E1813 and D17092–4. miRNA has a pivotal role in regulating the expression of over 30% of protein-coding genes by its interaction with mRNA5. Given the impact of DICER1 in post-translational events, it is not entirely surprising that functional DICER1 is essential for vertebrate development as evidenced by developmental arrest and death of the embryo when both alleles are lost6,7. Conceptually, DICER1 can be regarded as either a tumor suppressor gene due to loss-of-function mutations or an oncogene due to gain-of-function mutations; it is thought to function as a haploinsufficient tumor suppressor gene with the loss of one allele leading to tumor progression but loss of both alleles having an inhibitory effect for tumor development implying that one intact allele is needed for cell survival8.A study led by one of the authors (DAH) identified germline loss-of-function DICER1 mutations affecting the RNase IIIb domain in affected families with pleuropulmonary blastoma (PPB)9, a rare dysembryonic lung malignancy of childhood which was not the only manifestation of this familial tumor predisposition syndrome; germline and somatic DICER1 mutations were subsequently identified in several other familial associated tumors in several extrapulmonary sites (Table 1). Individuals with germline DICER1 mutations also had non-neoplastic conditions including macrocephaly, renal structural anomalies, retinal abnormalities, dental perturbations, and the GLOW syndrome (global developmental delay, lung cysts, overgrowth and Wilms tumor). These associations encircle the DICER1 tumor predisposition syndrome (Online Mendelian Inheritance in Man numbers 606241, 601200 and 138800), with the estimation that 90% of those affected by this syndrome inherited a germline mutation from one of their parents, with a pattern of autosomal dominant inheritance10.