NTRK1 and neoplasm: There are three main purposes for FISH in the NTRK study algorithm [8, 15]: (i) to confirm or rule out NTRK fusions in cases with IHC positivity; this is done by performing three FISH procedures, one each for NTRK1, NTRK2, and NTRK3; (ii) to confirm nuclear IHC positivity for NTRK3; and (iii) to confirm NTRK fusion in a neoplasm whose histology predicts that type of fusion (e.g. a FISH for NTRK3 in a secretory breast carcinoma).