It is believed that approximately one thousand Space Marine Chapters exist at all times though extremely violent warzones, attrition, space phenomena and other factors change the number of Chapters so radically no one can track them accurately. Over half (60%) of all Space Marine Chapters today are descendants of Ultramarines genestocks.,{{Fn|11c}}{{Fn|11d}} by the start of the 41st millennium, this percentage had increased to over two-thirds (67%) of chapters.{{Fn|76a}}