P3X63Ag8.653 is a subclone P3X63.Ag8 which was originally selected based with 8-aza-guanine sensitivity. Periodic growth in 8-asa-guanine ensures cells are HGPRT negative and thymidine kinase negative. Growth on 6-Thio-Guanine is useful for periodic selection for HAT sensitivity. Also remember that cells for fusion should not be passaged for a long times so keep frozen stocks and do not maintain the cells for more than several passages prior to fusion. It is also good practice to dilute the myeloma cells to about 200,000 cells/ml the day before fusion. Finally, I think your viability should be greater than 70% after you thaw the cells. Try bringing a cup of 37C water to your frozen cells and thawing as quickly as possible at 37C until the cells are completely thawed.