The HMGIC gene, located in chromosome band 12q15, is rearranged in many different benign human tumors, often resulting in its fusion to ectopic sequences from other genes. The t(3;12)(q27;q14-q15) fuses HMGIC with the LPP gene and has so far been described exclusively in lipomas. Thus, it can be hypothesized that this particular gene fusion determines the adipocytic differentiation. We studied five pulmonary chondroid hamartomas all showing a t(3;12)(q27;q14-q15) that apparently was identical to the one observed in lipomas. By fluorescence in situ hybridization we found that both HMGIC and LPP are disrupted by this translocation. By RT-PCR the existence of a HMGIC/LPP fusion gene was confirmed. These results show that the fusion is not specific for lipomas. We favor the hypothesis that it is an ectopic sequence fused to HMGIC that is responsible for a cell shift to an embryogenic stage. Following this hypothesis the phenotype of the tumor may be induced by extracellular signal transduction.