Let F(x) be a rational function of degree at least two with coefficients in Q. A theorem of Northcott[2] says that F has only finitely many Q-rational preperiodic points, i.e., F has only finitely many preperiodic points in P1(Q). The Uniform Boundedness Conjecture[3] of Morton and Silverman says that the number of preperiodic points of F in P1(Q) is bounded by a constant that depends only on the degree of F.