The authors wish to acknowledge technical assistance from: D. Godfrey, S. Lionikaite, V. Lionikaite, A.S. Lionikiene and J. Zekos as well as technical and intellectual input from M. Abney, J. Borevitz, K. Broman, N. Cai, R. Cheng, N. Cox, R. Davies, J. Flint, L. Goodstadt, P. Grabowski, B. Harr, E. Leffler, R. Mott, J. Nicod, J. Novembre, A. Price, M. Stephens, D. Weeks and X. Zhou. This project was funded by NIH R01GM097737 and P50DA037844 (A.A.P.), NIH T32DA07255 (C.C.P.), NIH T32GM07197 (N.M.G.), NIH R01AR056280 (D.A.B.), NIH R01AR060234 (C.L.A.-B.), the Fellowship from the Human Frontiers Science Program (P.C.) and the Howard Hughes Medical Institute (J.K.P.).