The availability of restriction enzymes which cleave at unique DNA sequences, have made it possible to physically map genes and isolate specific DNA fragments for sequence analysis. Specific gene sequences can be used to measure mRNA levels in cells and are thus useful tools for studying gene regulation. The isolation and characterization of the tissue-type plasminogen activator (t-PA) gene was begun in 1980. The translational product was characterized as t-PA by several criteria, including measurement of biological activity, immunological cross-reactivity, and fibrin stimulation. Human t-PA mRNA from melanoma cells was also identified by translation of RNA in a rabbit reticulocyte lysate system, supplemented with dog pancreas microsomes. A t-PA cDNA clone has also been isolated from phorbol ester stimulated HeLa cells. The system for activation of plasminogen includes other proteins besides t-PA, that neutralizes and activates t-PA activity and which may therefore be important for the regulation of the generated PA-activity.