This Human PAI-1 is a collection of four mutations: K154T, Q139L, M354I and H150H. These mutations offer great stability while locking PAI-1 into its active conformation. The N-terminus has been additionally modified with the introduction of a 6-X Histidine tag. The tag allows for the immobilization of functionally active PAI-1 onto surfaces such as metal chelate microtiter plates or Ni⁺² resins.