Conspicuous By It's Absence

While I reel in the insanity of my country's actions, let me offer a diversion. Examine all numbers between 1 and 100; there is something conspicuous by it's absence. It is overt and it is obvious--once you know what it is. Until you do, this question rates as a first class brain teaser. Rereading "No More Than Four," (6-17-16) may extend your normal, everyday perception, and aid in solving the riddle.