I. Cycles in simple genetic systems: The space of selection matrices Eigenvalues, eigenvectors and linear differential equations Nonlinear equilibria and Hopf bifurcations The Hopf bifurcation locus Parametrization of $\Sigma_H$ Symmetries Concluding remarks; II. Parametric description of the Hopf locus: Linear maps, bilinear forms and matrices Coordinates and the cylindrical frame Linearizations and Hessians The parametrization Symmetric cases MARMC Asymptotic estimates; III. Programs and examples: The general program The symmetric program Sample computations Bibliography.