  A strained and undoped HgTe layer is a three-dimensional topological
insulator, in which electronic transport occurs dominantly through its surface
states. In this Letter, we present transport measurements on HgTe-based
Josephson junctions with Nb as superconductor. Although the Nb-HgTe interfaces
have a low transparency, we observe a strong zero-bias anomaly in the
differential resistance measurements. This anomaly originates from
proximity-induced superconductivity in the HgTe surface states. In the most
transparent junction, we observe periodic oscillations of the differential
resistance as function of an applied magnetic field, which correspond to a
Fraunhofer-like pattern. This unambiguously shows that a precursor of the
Josephson effect occurs in the topological surface states of HgTe.
