The Mozilla Public License (MPL) is a free and open source software license developed and maintained by the Mozilla Foundation. It is a weak copyleft, characterized as a middle ground between permissive free software licenses and the GNU General Public License (GPL), that seeks to balance the concerns of proprietary and open source developers. As such, it allows re-licensing. MPL software can thus be converted into a copyleft license such as the GPL, or to a proprietary license (example:KaiOS).