An almost perfect nonlinear (APN) function (necessarily a polynomial function) on a finite field F is called exceptional APN, if it is also APN on infinitely many extensions of F. In this article we consider the most studied case of F = F 2 n . A conjecture of Janwa-Wilson and McGuire-Janwa- Wilson (1993Wilson ( /1996, settled in 2011, was that the only monomial exceptional APN functions are the monomials x n , where n = 2 k + 1 or n = 2 2k − 2 k + 1 (the Gold or the Kasami exponents, respectively). A subsequent conjecture states that any exceptional APN function is one of the monomials just described. One of our results is that all functions of the form f (x) = x 2 k +1 + h(x) (for any odd degree h(x), with a mild condition in few cases), are not exceptional APN, extending substantially several recent results towards the resolution of the stated conjecture. We also show absolute irreducibility of a class of multivariate polynomials over finite fields (by repeated hyperplane sections, linear transformations, and reductions) and discuss their applications.