  We discuss the discrete Z_k D-brane charges (twisted K-theory charges) in
five-brane backgrounds from several different points of view. In particular, we
interpret it as a result of a standard Higgs mechanism. We show that certain
degrees of freedom (singletons) on the boundary of space can extend the
corresponding Z_k symmetry to U(1). Related ideas clarify the role of AdS
singletons in the AdS/CFT correspondence.
