def _inject_mutations_1D_X(phi, dt, xx, theta0, beta, alpha):
    """
    Inject novel mutations for a timestep.
    """
    factor = 2.0 / (1.0 + 2.0 * beta) * (1.0 / (alpha + 1.0) + beta)
    phi[1] += dt / xx[1] * theta0 / 2.0 * factor * 2.0 / (xx[2] - xx[0])
    return phi
