  We propose several quark mass deformations of the holographic model of
massless QCD using the D4/D8/D8bar brane configuration proposed by Sakai and
Sugimoto. The deformations are based on introducing additional D4 or D6 branes
away from the QCD D4 branes. The idea is similar to extended technicolor
theories, where the chiral symmetry breaking by additional D-branes are
mediated to QCD to induce non-zero quark masses. In the D-brane picture as well
as the holographic dual gravity description, the quark and the pion masses are
generated by novel worldsheet instantons with finite area. We also derive the
Gell-Mann-Oakes-Renner relation, and find the value of the chiral condensate in
the Sakai-Sugimoto model.
