We argue that double logarithmic corrections αsln2x need to be
resumed in perturbative QCD factorization theorem for exclusive B meson
decays, when the end-point region with a momentum fraction x→0 is
important. These double logarithms, being of the collinear origin, are absorbed
into a quark jet function, which is defined by a matrix element of a quark
field attached by a Wilson line. The factorization of the jet function from the
decay B→γlνˉ is proved to all orders. Threshold resummation for
the jet function leads to a universal, {\it i.e.}, process-independent, Sudakov
factor, whose qualitative behavior is analyzed and found to smear the end-point
singularities in heavy-to-light transition form factors.Comment: 10 pages, more details are include