The first gene fusions to be defined in solid cancers, RET/PTC [15] and NTRK1 [16] rearrangements in papillary thyroid carcinoma were identified through a “transformation assay” using cancer genomic DNA transfected into murine NIH3T3 cells, followed by retrieval and analysis of human genomic DNA from transformed cells [17]. The gene discussed is NTRK1; the disease is cancer.