Collen collaborated in the same period with Professor Alfons Billiau of the Rega Institute for Medical Research in Leuven, Belgium on the inhibition of plasminogen activators secreted by malignant cells in culture, hypothesizing that synthetic inhibitors might suppress the malignant phenotype of these cells. In order to have a source for these "malignant plasminogen activators" the Bowes Melanoma Cell line was obtained from Professor Daniel Rifkin of the Rockefeller University, New York City at the end of 1978. It quickly appeared that this cell line produced large amounts of a plasminogen activator with a molecular weight of 70,000 dalton and a high affinity for fibrin, characteristic of human tissue-type plasminogen activator (t-PA), whereas most other malignant cells produced a plasminogen activator with a molecular weight of 54,000 dalton without affinity for fibrin, which is characteristic of pro-urokinase. This game-changing, although trivial serendipitous observation, made on 9 February 1979, was to change the course of thrombolytic therapy.