Apo (a) consists of multiple tandem repeat of kringle 4, which resembles a counterpart of plasminogen. Plasma Lp (a) levels are genetically determined primarily by alleles at the apo (a) gene. Apo (a) shows size heterogeneity on the analysis of the protein and the mRNA. Pulse field gel electrophoresis revealed that size heterogeneity is largely due to different numbers of kringle 4-encoding sequences in the apo (a) gene. The cloning of apo (a) gene and identification of the 5'-flanking promoter region provide tools to study the regulation of apo (a) gene. The development of transgenic mice expressing human apo (a) offered a good model for understanding of atherosclerosis associated with elevated plasma levels of Lp (a).