since this is nothing but a Clebsh-Gordan coefficient occurring in the conformal partial wave expansion. It is still unclear if the skewness effect (76) is an indispensable feature of GPD phenomenology (see discussion in [14, 38]). Let us emphasize that the skewness ratio in the framework of the dual parametrization is not with necessity given by (76). In order to alter the skewness ratio one has to take into account the contribution of subsequent forward like functions Q$_{2}$$_{ν}$ ( G$_{2}$$_{ν}$ ) singular enough to change the small ξ asymptotic behavior of H $^{q g}$( ξ, ξ ). For this one has to assume that Q$_{2}$$_{ν}$ ( x ), G$_{2}$$_{ν}$ ( x ) ∼ 1 /x 2 ν + α $^{q,g}$. The rigorous way to handle the occurring divergencies of the generalized form factors $^{B$_{2}$$_{ν}$}$− 1 0 and of the quark and gluon D -form factors was described in details in [22]. In the remaining part of this section we consider a toy model in order to briefly sketch the possible fitting strategy for hard exclusive scattering observables based on the dual parametrization of GPDs. For definiteness we discuss the case of DVCS at the leading order. In order to fit the experimental data we have to propose certain Ansatz for the relevant GPD quintessence functions N ( x, t ). The contribution of the forward like function Q$_{0}$ ( x, t ) into GPD quintessence function is entirely fixed in terms of the t -dependent parton dis- tributions. Since for the moment we discuss DVCS let q ( x, t ) be the DVCS combination of t -dependent singlet parton distributions q ( x, t ) = 4 $_{9}$u$_{+}$ ( x, t ) + 1 $_{9}$d$_{+}$ ( x, t ) . The observable quantities can be expressed in terms of the standard elementary amplitude where H ( x, ξ, t ) is the DVCS combination of singlet quark GPDs. The leading singular behavior for x ∼ 0 of the t -dependent PDF q ( x, t ) is assumed to be determined by the linear Regge trajectory a ( t ) ≡ α + α ′ t : B q ( E, M ) 2 ν − 1 0 ( t ) = ∫ 1 0 dxx 2 ν − $^{1}$Q ( E, M ) 2 ν ( x, t ) ; B g ( E, M ) 2 ν − 1 0 ( t ) = 1 0 dxx 2 ν − $^{1}$G ( E, M ) 2 ν ( x, t ) (78) A ( ξ, t ) = ∫ 1 0 dxH ( x, ξ, t ) [ 1 ξ − x − iϵ − 1 ξ + x − iϵ ] , (79) q ( x, t ) ∼ c q 1 x a ( t ) , (80) 24