We report the effect of cellular antisense therapy using a new transporter structure to suppress pseudoexon activation in primary dermal fibroblasts from patients with mutations in the PTS gene encoding 6pyruvoyltetrahydropterin synthase (PTPS), and also a deeper functional analysis of the mutant change c.164-712A>T that activates the exonization of an AluSq sequence previously reported. The results obtained using ex vivo minigenes showed that the change c.164-712A>T at position +6, but also at position +3 of the pseudoexon (c.164-715 T>A), seems to strengthen the pseudoexon 5´splice site thus improving complementarity to U1 snRNA. We have demonstrated that even though two different 3´ss could be used to insert a 70-bp or 45-bp pseudoexons, these changes predominantly activate the most upstream one. Regarding the use of a new vehicle effective for in vivo delivery into a wide variety of tissues, we have used a conjugates of an octa-guanidine dendrimer covalently linked to the morpholino oligos to perform a parallel set of experiments. The RT-PCR and also the pterin profile showed that the AMO transporter-conjugates successfully corrected the mis-splicing recovering the metabolic pathway and confirm the therapeutical option for this type of mutations. Background: The main treatment for PKU is lifelong dietary phenylalanine restriction. Early dietary treatment is effective in hyperphenylalaninemia, but this Phe restricted diet has negative aspects. A subset of patients shows a clinically significant reduction in blood phenylalanine when treated with pharmacological doses of Sapropterin. Sapropterin has been approved for the treatment of hyperphenylalaninaemia in patients>or= 4 years of age.