A cDNA clone coding for a human brain protein (HuD), which is highly homologous to the Drosophila neuronal determination protein Elav and elicits antibody formation in a high proportion of patients with paraneoplastic encephalomyelitis, was used to isolate a λ phage recombinant clone, including a large fragment of the relevant human genomic region. The fragment proved to be an efficient probe for the precise subregional mapping of the gene by molecular in situ hybridization onto extended human prometaphase chromosomes. Analysis of banded metaphases with clear hybridization signals pointed unequivocally to the localization of the HuD gene to human chromosome band 1p34.