We show that a heterozygous germline mutation c.4031C>T (p.S1344L) in the RNase IIIa domain of DICER1 is compatible with life, and causes a complex ‘DICER1 syndrome plus’ phenotype with extensive bilateral and multilobar lung cysts, PPB, cystic nephroma, Wilms tumour, well-differentiated fetal lung adenocarcinoma, ID, macrocephaly, ear pits and other physical anomalies. The gene discussed is DICER1; the disease is lung adenocarcinoma.