then it remains in the MN2 class.

The problem of reaching a satisfactory analytical understanding, and even that of obtaining sound results from numerical integrations of the continuous Langevin-equation (in either the interface or the density language) representative of this class remains an open challenge.

Summing up, even though strong evidence is provided confirming the existence of a universality class ([*i.e.* ]{} the corresponding critical exponents are computed with good precision in one dimension and they are universal in two different discrete models), its theoretical description in terms of Langevin equations, contrarily to what happens for the closely related MN1 class, is far from satisfactory. In particular, the Langevin equation does not seem to admit sound mean-field solutions, nor is amenable to be treated by means of standard perturbative field theoretical tools, nor it admits a stable numerical integration. Identifying the physical causes at the root of these difficulties is a challenge for future research.

**ACKNOWLEDGMENTS**

F.S. acknowledges financial support from the Fundação para a Ciência e a Tecnologia, contract SFRH/BPD/5654/2001. Financial support from the Spanish MCyT (FEDER) under project BFM2001-2841, and from the AECI, are also acknowledged.

See J. Garc[í]{}a-Ojalvo, and J. M. Sancho, [*Noise in Spatially Extended Systems*]{}, 