The central hypothesis of this application is that posterolateral spine fusion can be successfully achieved with a novel and simple minimally invasive percutaneous injection. We propose that this can be done by injection of AdBMP2 transduced human fibroblasts possessing an icasp9M that have been micro-encapsulated with osteoclast degradable hydrogel into the desired fusion site. During the course of this grant application we observed that in the rat model, we were unable to induce the heterotopic bone formation necessary to fuse the spine. Since this is a major stumbling block for the development of this therapy, we compared the process in mice to rats and found thatMMP9 activation was not active, but rather existed as in the proform. This is very different from mice or even humans where all MMP9 at the site of HO is active. Additionally, we confirmed that the lack of bone formation after delivery of the microspheres to larger animal models was not the result of an immune response to the adenovirus transduced cells encapsulated in the PEG-Da hydrogels. The results showed that mice previously exposed to adenovirus transduced cells totally suppressed the bone formation ability of the AdBMP2transduced cells, however, if they were encapsulated in the PEG-Da hydrogel, then bone formation proceeded as expected. Thus the hydrogel can overcome cellular immunity associated with the materials. Finally, we have shown that we can attenuate the bone formation through inclusion of a stably integrated icasp9M that leads to rapid apoptosis in the hydrogel encapsulated cells in vivo, after exposure to a chemical inducer of dimerization CID. The work is the focus of 4 manuscripts.