REXIS was originally scheduled for installation in September 2015. However, after careful consideration, the project decided that we could not approve moving forward with an ATLO Integration Readiness Review (AIRR) for the REXIS instrument at that time. We came to this conclusion based upon the number of items that remained open on the instrument. REXIS has always been primarily viewed as an educational opportunity to train the next generation of scientists and engineers in the development of spaceflight hardware—so, science return from the instrument is a second priority to ensuring that REXIS does not damage the spacecraft or other science instruments. To this end, the project focused on the “do-no-harm” verification items that needed to be successfully closed before we could be assured that the REXIS instrument was safe to integrate to the spacecraft. In addition, there was an unexplained shift in a primary vibration mode that was revealed during instrument-level vibration testing. That unexplained shift had our mechanical engineers concerned about the safety of the instrument.