PyMOL is a free cross-platform molecular graphics system made possible through recent advances in hardware 1, internet 2, and software development technology 3. PyMOL provides most of the capabilities and performance of traditional molecular graphics packages written in C or Fortran 4. However, its integrated Python interpreter endows it with features and expandability unmatched by any traditional package. PyMOL has been released under a completely unrestrictive open-source software license 5 so that all scientists and software developers can freely adopt PyMOL and then distribute derivative works based on it without cost or limitation.