In this paper, we describe labeling and hybridization conditions for oligonucleotide probes that detect human triplet repeat sequences in yeast artificial chromosomes (YACs). Restriction digests of YACs containing the CAG repeat sequence of the SCA1 gene were used as positive controls. Several hybridization mixtures and temperatures and two different labeling techniques were tested in order to determine optimal conditions. CAG, CGG, AGG, and ATT repeat sequences were mapped on YACs from a contig in 6p23, where SCA1 is located.