INSM1 is a protein containing both a zinc finger DNA-binding domain and a putative prohormone domain. INSM1 was originally isolated from an insulinoma subtraction library (1). Subsequent studies revealed that INSM1 expression is quite specific for neuroendocrine tumors and is absent from other non-neuroendocrine tumors. An extensive neuroendocrine tumor marker analysis in lung cancer demonstrated that 97% (30/31) of small cell lung carcinoma cell lines are positive for INSM1 expression. INSM1 expression also appears in nonsmall cell lung carcinoma with a neuroendocrine phenotype. INSM1 showed a high concordance with the other neuroendocrine markers l-dopa decarboxylase and chromogranin A. In addition, several microarray gene expression profiling experiments of small cell lung cancer cells identified INSM1 as a highly specific marker that is abundantly expressed in neuroendocrine lung tumors. INSM1 was identified as a prominent differential marker for SCLC along with Hash1 and gastrin-releasing peptide.