- Association of a single nucleotide polymorphism in SPP1 with growth traits and twinning in a cattle population selected for twinning rate. Allan, M.F., Thallman, R.M., Cushman, R.A., Echternkamp, S.E., White, S.N., Kuehn, L.A., Casas, E., Smith, T.P. J. Anim. Sci. (2007)