Animal nuclear genomes contain DNA sequences of various degrees of repetition. These sequences are organized in highly ordered fashions; repetitive and nonrepetitive sequences either alternate in short periods, i.e., short [0.2-0.4 kilobases (kb) long] repeats are flanked by nonrepetitive sequences less than 2 kb long, or in longer periods, with repetitive and/or nonrepetitive sequences extending for several kilobases. There are two main categories of genome organization, namely those exhibiting short-period interspersion and those that do not. There are arguments for and against a regulatory role of short interspersed repetitive sequences. Besides the merely 'statistical' kinetic approach by conventional reassociation kinetics, sequence organization has been studied by restriction endonuclease mapping and nucleotide sequencing. Such studies have revealed some general features of the organization of the eukaryotic gene and its transcripts, namely possible 'promoters', 'leaders', 'introns', 'exons', 'flanking sequences', 'caps', ribosome-binding sites, and poly(A) sequences. This paper discusses how these elements of a gene might serve regulatory roles in its expression.