  We report two examples of transport phenomena based on sharp features in the
effective density of states of molecular-scale transistors: Kondo physics in
C$_{60}$-based devices, and gate-modulated negative differential resistance
(NDR) in ``control'' devices that we ascribe to adsorbed contamination. We
discuss the need for a statistical approach to device characterization, and the
criteria that must be satisfied to infer that transport is based on single
molecules. We describe apparent Kondo physics in C$_{60}$-based single-molecule
transistors (SMTs), including signatures of molecular vibrations in the Kondo
regime. Finally, we report gate-modulated NDR in devices made without
intentional molecular components, and discuss possible origins of this
property.
