A complex disease or trait is a phenotype that is produced as a result of genetic risk factors, environmental factors and possibly their interactions. Most common diseases and traits are complex in nature. Numerous genetic variants that contribute to the development of complex diseases and traits have been identified by genome-wide association studies. Subsequent identification and characterization of genuine causal variants have been proved very challenging and remain a bottleneck in the understanding of complex diseases. Among novel tools of synthetic biology useful of functional genomics study of putative causal variants is the massively parallel reporter assay (MPRA) and its modifications. This short overview will mainly introduce MPRA and its various aspects from the perspective of finding and characterizing casual variants. Two other massively parallel assays will also be touched on briefly.