Macintosh Common Lisp (MCL) is an implementation and IDE for the Common Lisp programming language. Various versions of MCL run under the classic Mac OS (m68k and PPC) and Mac OS X. Versions of MCL up to and including 5.1 are proprietary. Version 5.2 has been open sourced. In 2009 a new different version of MCL has been open sourced: RMCL. RMCL is based on MCL 5.1 and does run under Rosetta on Intel-based Macs.