Ríos, J. C., J. G. McEwen, A. Cuartas, F. Cuesta, and S. Parra. “Role of Four Polymorphisms of the β-2 Adrenergic Receptor Gen in Patients With Preterm Delivery”. Colombia Medica, vol. 39, no. 1, Mar. 2008, pp. 7-15, doi:10.25100/cm.v39i1.545.