The human cytidine-5'-triphosphate synthetase (CTPS) gene was mapped by a direct mapping system combined with fluorescence in situ hybridization and replicated prometaphase R-bands. By high-resolution banding analysis, the signals were localized to band 34.1-34.3 of the short arm of chromosome 1; 1p34.1-p34.3. Simple procedures for the detection of R-bands are described.