MBF (Marine's Best Friend) is a source port created by Lee Killough after he left the Boom team. It is regarded by some as Boom's successor. As with Boom, MBF was limited to running under MS-DOS. MBF adds several features: MBF was originally licensed under the non-free Doom Source License, but Killough has since relicensed his work under the GNU GPL. MBF formed the basis of the SMMU port by Simon Howard. In 2004 it was ported to Windows with the name WinMBF by Team Eternity.