Microbead expression arrays

Strategies for expression analysis range from exhaustive sequencing (and thus counting) of cDNAs to hybridization arrays. In the June issue of Nature Biotechnology Brenner et al. describe a method that combines the digital precision of the former with the speed and throughput of the latter (Nat. Biotech. 2000, 18:630-634). Brenner et al. attach tagged cDNAs to microbeads and then sequence the overhanging ends of the cDNAs by detecting the hybridization of fluorescently labeled probes. After one overhang is identified, a binding site for a type IIs restriction endonuclease (within the probe) is used to cleave a distant cleavage site (within the cDNA sequence) to expose a new overhang. The coming and going of fluorescent probes is monitored by confocal microscopy of the microbeads, which are immobilized in a flow cell. Hundreds of thousands of mRNAs are identified in a few days, exceeding the throughput per machine of conventional sequencers by over 10-fold.