The genes for the M2 subunit of ribonucleotide reductase (RRM2), ornithine decarboxylase (ODC1), and 55,000-Daltons protein (P5), are amplified in hydroxyurea-resistant hamster and human cell lines. These genomic sequences have been mapped to hamster chromosome 7 and to human chromosome 2p24-25 near the cytogenetic location of the N-myc gene. We now report that genomic sequences homologous to N-myc are amplified in hydroxyurea-resistant hamster lung cell line, 600H, and the N-myc gene segregates with hamster chromosome 7 in mouse-hamster somatic cell hybrids. The conserved linkage group consisting of the RRM2, ODC1, P5, and N-myc in the hamster and human genomes prompted our investigation of human neuroblastomas. We report here that genomic DNA from 1 of 6 primary neuroblastoma tumors containing amplified N-myc also contains amplified sequences homologous to a hamster ODC cDNA.