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Test data propagation through modules and test vector translation are two major challenges encountered in hierarchical testing. We propose a new synthesis-for-test approach in which multiplexers are embedded in the behavioral models of the various modules constituting a hierarchical system. This approach can also be applied to system-on-a-chip designs in which synthesizable models are available for the embedded cores. The embedded multiplexers provide complete single-cycle transparency, thereby offering a straightforward yet effective solution to the problems of test data propagation and test vector translation. In order to determine module I/O bitwidths for single-cycle transparency, a global analysis is carried out using a graph-theoretic framework and an optimization method based on integer linear programming. Case studies using high-level synthesis benchmarks and an industrial-strength benchmark show that synthesis for transparency introduces very little area and performance overhead.