  The approximately symmetric form of the PMNS matrix suggests that there could
exist a hidden symmetry which makes the PMNS matrix different from the CKM
matrix for quarks. In literature, all the proposed fully symmetric textures
exhibit an explicit $\mu-\tau$ symmetry in addition to other symmetries which
may be different for various textures. Observing obvious deviations of the
practical PMNS matrix elements from those in the symmetric textures, there must
be a mechanism to distort the symmetry. It might be due to existence of light
sterile neutrinos. In this work, we study the case of the Tribimaximal texture
and propose that its apparent symmetry disappears due to existence of a sterile
neutrino. We observe that introducing just one sterile neutrino is still not
sufficient to recover the data, thus a slight $\mu-\tau$ symmetry breaking is
also needed. By considering those factors, we obtain the PMNS matrix elements
which are consistent with data within the experimental tolerance.
