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A theoretical analysis has been performed of the HERMES semi-inclusive deepinelastic data on the difference between $\pi^{+}$ and $\pi^{-}$multiplicities. It turns out that the application of a standard perturbativeQCD analysis, using, as usual, isospin symmetry for the fragmentationfunctions, leads to a poor description of the deuteron data at small z values.If one allows a breaking of isospin invariance, a good fit to both proton anddeuteron data can be achieved for all measured z, but the level of isospinviolation, especially at small values of z, is simply not credible. We suspectthat the problem is a consequence of using the factorized QCD treatment in akinematic region where it is unjustified.

Problems with the perturbative QCD interpretation of HERMES data on semi-inclusive lepto-production of pions