An experimental form of DNA array in which the probes are not identified by dint of being deposited in a fixed position on a surface. MPSS can be considered among this brethen.

Silica microbeads are held in suspension (1) or distributed randomly across a surface (2). Each carries multiple copies of a single probe, with all microbeads that bear the same probe containing the same 'optical barcode' (unique ratios of two or more fluorescent dyes absorbed inside). Bound target is detected by its fluoroscent tag (usually; alternatives exist), and the identities of the lucky microbeads (and hence of their attached probes) is determined by interrogating the barcodes via either flow cytometry (FACS) (1) or multispectral imaging (CCDs) (2).

Current implementations are little more than fishing expeditions for mere handfuls of genes, but the technique is in its early stages. Rather than use optical barcodes, it may be easier to take a page out of MPSS's book and interrogate oligonucleotide tags attached to each microbead.

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