"Patentleft (also patent left, copyleft-style patent license) is the practice of licensing patents (especially biological patents) for royalty-free use, on the condition that adopters license related improvements they develop under the same terms. Copyleft-style licensors seek "continuous growth of a universally accessible technology commons" from which they, and others, will benefit.
"The Biological Innovation for Open Society (BIOS) project implemented a patentleft system to encourage re-contribution and collaborative innovation of their technology. BIOS holds a patented technology for transferring genes in plants, and licenses the technology under the terms that, if a license holder improves the gene transfer tool and patents the improvement, then their improvement must be made available to all the other license holders." (http://en.wikipedia.org/wiki/Patentleft)