Furthermore, it has previously been shown that two different oligonucleotides of 18 nt, complementary to the inner and outer portions of a 25 nt probe, could hybridize in solution with equal efficiency but, when the probe was coupled to a solid phase via a C6 linker, the 18-nt target complementary to the outer part of the 25-nt probe bound more efficiently than the 18-nt target complementary to the inner part, close to the solid phase (37-39), cf (33) . On our probes, the 5 0 end was coupled to the microspheres via an amino C12 linker. We did not observe any significant differences between matching stretches close to the bead surface and far from it and conjecture that perhaps the long linker allowed for greater accessibility.