The neuron-restrictive silencer factor [NRSF (RE-1 silencing transcription factor/X box repressor)] is a transcriptional silencer,which we have previously implicated in deregulation of the vasopressin promoter in small cell lung cancer (SCLC). Here we describe a novel splice variant of the NRSF transcript, which is highly expressed in SCLCs. The variant was detected in both established cell lines and primary SCLC cultures as well as in some primitive neuroectodermal tumor biopsies. It was present at very low levels in human brain tissue, non-SCLC tumors, and normal bronchial epithelium. This human splice variant, which is massively overexpressed in SCLCs, incorporates a 50-bp insert between exons 5 and 6, introducing a stop codon and predicting translation of a truncated NRSF isoform. We propose that the encoded isoform may antagonize repression of the vasopressin promoter and other “neuronal” genes with neuron- restrictive silencer elements in SCLCs. Thus, up-regulated expression of this NRSF isoform may be a key early factor in defining the neuroendocrine phenotype of these tumors. The NRSF splice variant represents a specific clinical marker that could prove useful in detection of the majority of SCLCs.