Yukawa textures in string unified models with SU(4)circle times O(4) symmetry

We discuss the origin of Yukawa textures in the string-inspired and string-derived models based on the gauge group SU(4) x SU(2)(L) x SU(2)(R) supplemented by a U(1)(X) gauged family symmetry. The gauge symmetries are broken down to those of the minimal supersymmetric standard model which is the effective theory below 10(16) GeV. The combination of the U(1)(X) family symmetry and the Pati-Salam gauge group leads to a successful and predictive set of Yukawa textures involving two kinds of texture zeros: horizontal and vertical texture zeros. We discuss both symmetric and nonsymmetric textures in models of this kind, and in the second case perform a detailed numerical fit to the charged fermion mass and mixing data. Two of the Yukawa textures allow a low energy fit to the data with a total chi(2) of 0.39 and 1.02, respectively, for three degrees of freedom. We also make a first attempt at deriving the nonrenormalizable operators required for the Yukawa textures from string theory.