(24,000 RefSeq + 24,000 supplementary)

The bead • Each silica bead is 3 microns in diameter • 700,000 copies of same probe sequence are covalently attached to each bead for hybridisation & decoding • Can have more than one bead for a particular gene * Beads in wells • Bead pools produced containing 384 to 24,000 bead types • Wells created in either fibre optic bundle (hexagon) or chip (rectangle) • Beads self-assemble into wells to form randomly arranged array of beads • Average of 30 beads of each type (adds robustness) • Each array produced separately Bead preparation and array production Decoding process • Beads get assigned random locations on each array. Need to know what type every bead is • Decoding is achieved by series of sequential hybridisations* • Each bead type defined by unique DNA sequence that is recognised by a complementary decoder • Process is highly effective (error rate < 1 x 10-4 per bead) * Gunderson et al. Decoding randomly ordered DNA arrays