Xeroderma pigmentosum group C (XPC) protein initiates the DNA excision repair of helix-distorting base lesions. To understand how this versatile subunit searches for aberrant sites within the vast background of normal genomic DNA, the real-time redistribution of fluorescent fusion constructs was monitored after high-resolution DNA damage induction. Bidirectional truncation analyses disclosed a surprisingly short recognition hotspot, comprising approximately 15% of human XPC, that includes two beta-hairpin domains with a preference for non-hydrogen-bonded bases in double-stranded DNA. However, to detect damaged sites in living cells, these DNA-attractive domains depend on the partially DNA-repulsive action of an adjacent beta-turn extension that promotes the mobility of XPC molecules searching for lesions. The key function of this dynamic interaction surface is shown by a site-directed charge inversion, which results in increased affinity for native DNA, retarded nuclear mobility and diminished repair efficiency. These studies reveal a two-stage discrimination process, whereby XPC protein first deploys a dynamic sensor interface to rapidly interrogate the double helix, thus forming a transient recognition intermediate before the final installation of a more static repair-initiating complex.