The authors present the two phases of Hoxd gene activation during limb development. In this process, the 5’ Hoxd genes show a striking collinear pattern of expression both spatially and temporally along the anteroposterior axis of fins and limbs. The existing models provide a detailed and accurate description of the gene expression features. I think, the next step should be to try to explain the above data as the outcome of a universal causing mechanism. To this end I have proposed an integrated mechanism acting at both the multicellular and subcellular levels resulting in the creation of forces that gradually pull the Hox chromatin fiber outside the chromosome territory (Papageorgiou (2006) Int. J. Dev. Biol. 50: 301-308). If the forces are electric they act like electrophoresis in vivo. Following this model the existing collinearity data are well reproduced. In this paper I propose further experiments that can directly test the electric hypothesis. I do not know whether these experiments are presently feasible.