We thank P. Burgoyne, P. Cohen, E. Alani, and P. Moens for helpful discussions, R. Viswanatha for helpful comments on the manuscript, W. Pawlowski and T. Pawlowska for input on the evolutionary tree of PCH2, R. Freire and J. Chen for antibodies, M. Jasin for Spo11 − embryonic stem cells, and R. Munroe for generating chimeric mice. We also thank an anonymous reviewer for pointing out that Pch2p might be sensing synaptic polymerization in yeast, not initiation.