The invention provides methods for determining the activity of a plurality of nucleic acid regulatory elements. These methods may facilitate, e.g., the systematic reverse engineering, and optimization of mammalian cis-regulatory elements at high resolution and at a large scale. The method may include integration of multiplexed DNA synthesis and sequencing technologies to generate and quantify the transcriptional regulatory activity of e.g., thousands of arbitrary DNA sequences in parallel in cell-based as says (e.g., mammalian cell based assays).