federal buffalo herds had some cattle DNA. With the advent of nuclear microsatellite DNA testing, the number of herds known to contain cattle genes has increased. Though approximately 500,000 bison exist on private ranches and in public herds, some people estimate that perhaps only 15,000 to 25,000 of these bison are pure and are not actually bison-cattle hybrids. "DNA from domestic cattle ( Bos taurus ) has been detected in nearly all bison herds examined to date." [ 60 ] Significant public bison herds that do not appear to have hybridized domestic cattle genes are the Yellowstone Park bison herd , the Henry Mountains bison herd which was started with bison taken from Yellowstone Park, the Wind Cave bison herd and the Wood Buffalo National Park bison herd and subsidiary herds started from it, in Canada. A landmark study of bison genetics that was performed by James Derr of the Texas A&M University corroborated this. [ 61 ] The Derr study was undertaken in an attempt to determine what genetic problems bison might face as they repopulate former areas, and it noted that bison seem to be doing quite well, despite their apparent genetic bottleneck . One possible explanation for this might be the small amount of domestic cattle genes that are now in most bison populations, though this is not the only possible explanation for bison success. In the study cattle genes were also found in small amounts throughout most national, state and private herds. "The hybridization experiments conducted