PRAD1 (previously D11S287) is a putative proto-oncogene at 11q13, activated by overexpression through gene rearrangement or gene amplification in several types of human tumors including parathyroid adenomas, centrocytic lymphomas and other B-cell tumors with t(11;14), and breast cancers. PRAD1 (also CCND1) encodes cyclin D1, which may regulate the G1-S phase transition in the cell cycle. Here, we report the cloning and characterization of the chromosomal PRAD1/cyclin D1 gene and the sequence of its promoter region. The gene spans about 15 kb and has 5 exons; its promoter region has Sp1 binding sites and no obvious TATA box, characteristics of housekeeping genes and growth-regulating genes. Furthermore, an E2F binding motif present close to the major transcription start site may be involved in cell cycle-dependent expression of this gene. We also report the sequence of DNAs spanning joining regions of a reciprocal parathyroid hormone/PRAD1 gene rearrangement in a parathyroid adenoma. Comparison with normal sequences suggests that the rearrangement was not a simple break-and-ligate event, but rather involved multiple steps, including two microdeletions and a microinversion. Very short sequences conserved near the breakpoints and symmetrical elements in the eventually inverted DNA segment might have played a role in this illegitimate complex recombination, which may have similarities with a constitutional translocation in Duchenne muscular dystrophy.