The entire first season (all 36 episodes) as well as the first 19 episodes of the second season are the only episodes in the public domain. The 20th episode of the second season to the end of the series are still under copyright protection. These 55 episodes ended up in the public domain because CBS, having bought the rights to the series shortly after its cancellation, neglected to renew their copyrights. As a result, these episodes have been unofficially released on home video and DVD on many low-budget labels. In many video prints of the public domain episodes, the original and much-loved theme music has been replaced by generic music due to copyright issues.