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The evolution of the collisionless tearing mode instability is studied using an electromagnetic gyrokinetic δf particle-in-cell simulation model. Drift-kinetic electrons are used. High resolution, small box (less than 10 ion gyroradii) simulations have been well benchmarked with eigenmode analysis and their nonlinear evolution agrees well with theory. In this regime, the ion response is not important and can be either fixed, adiabatic or fully gyrokinetic. Here, results are presented with larger box sizes (64 ion gyroradii radially) where the ion gyrokinetic response is important and cannot be neglected. In these larger box simulations, the instability exhibits an odd parity, different than the even tearing parity.