Avian myelocytomatosis virus MC29 is a replication-defective acute leukaemia virus which induces a variety of tumours in chickens including sarcomas, renal and hepatic carcinomas, and myelocytomatosis. The oncogenic potential of the virus is mediated by the gene v-myc, acquired from sequences (c-myc) present in normal uninfected chicken DNA. Sequences closely related to chicken c-myc have been highly conserved throughout evolution, from Drosophila to vertebrates. The hypothesis that c-myc may be involved in neoplastic transformation has been strengthened by the finding that B-cell lymphomas induced in chickens by avian leukosis virus (ALV) are often associated with increased expression of c-myc resulting from integration of the ALV provirus adjacent to the c-myc gene. More recently, it has been demonstrated that the malignant human cell line HL-60, derived from the peripheral blood leukocytes of a patient with acute promyelocytic leukaemia, expresses elevated levels of myc-related mRNA associated with an amplification of the c-myc gene. To explore the relationship of the human cellular myc gene with the corresponding viral oncogene from MC29, and to provide a framework for the analysis of the mechanism and significance of c-myc amplification in human tumours, we have isolated and determined the nucleotide sequence of a genomic clone prepared from a normal human library which contains all domains sharing homology with v-myc.