We have employed gene transfer to generate cell lines in which a chromosomal region consisting solely of defined DNA sequences has undergone gene amplification. We have analyzed recombinant clones from the amplified array to determine the physical structure of amplified DNA in the cell lines. The amplified DNA we have analyzed consists of a tandem array of at least 20 individual repeating units. The individual units are contiguous, and are joined to one another by homologous recombination between repeated sequences. At first approximation, all homologous recombinations are permitted such that crossing-over may occur between any two repeated sequences. Since individual units contain multiple repeated elements, the array is not a regularly repeating structure. The individual units within the array are heterogeneous, both in size and in sequence content. These observations suggest models of gene amplification which involve multiple cycles of unscheduled DNA replication at a single locus, followed by multiple recombination events which serve to link individual units to one another and ultimately to the chromosome.