The paper by Andreas Karch and Emanuel Katz explores the addition of flavors to the AdS/CFT correspondence, particularly focusing on the dual string representation of fundamental quarks in the context of QCD. They argue that flavors should be represented by space-time filling D-branes in a 5-dimensional closed string background. This requires several properties of D-branes in AdS, such as being able to end in thin air to account for massive quarks and coming in distinct sets to represent the chiral global symmetry. The authors demonstrate these properties using supersymmetric examples and show that the D-branes can be stabilized without carrying any charge, avoiding tadpole issues. They also discuss the probe limit, where the backreaction of the D-branes on the bulk geometry is neglected, and analyze the $\kappa$-symmetry on the worldvolume to ensure supersymmetry. The paper includes detailed analyses of specific examples, such as the D3-D7 system and the $\mathcal{N}=1$ SYM theory with flavors, and concludes with a discussion on the potential applications of these findings to non-supersymmetric large $N$ QCD.The paper by Andreas Karch and Emanuel Katz explores the addition of flavors to the AdS/CFT correspondence, particularly focusing on the dual string representation of fundamental quarks in the context of QCD. They argue that flavors should be represented by space-time filling D-branes in a 5-dimensional closed string background. This requires several properties of D-branes in AdS, such as being able to end in thin air to account for massive quarks and coming in distinct sets to represent the chiral global symmetry. The authors demonstrate these properties using supersymmetric examples and show that the D-branes can be stabilized without carrying any charge, avoiding tadpole issues. They also discuss the probe limit, where the backreaction of the D-branes on the bulk geometry is neglected, and analyze the $\kappa$-symmetry on the worldvolume to ensure supersymmetry. The paper includes detailed analyses of specific examples, such as the D3-D7 system and the $\mathcal{N}=1$ SYM theory with flavors, and concludes with a discussion on the potential applications of these findings to non-supersymmetric large $N$ QCD.