Earlier work has indicated that a region of 15 nt in a 50-mer probe or 20 nt in a 70-mer probe could cause significant cross hybridization in microarray hybridization experiments (30, 31) and our data agree with this. A 70-mer probe is able to hybridize with a region of 15 nt in an otherwise highly mismatching target in 3 M TMAC (74%15 nt, Figure 3A ). To summarize, the current study shows that a dInosine-free probe of 70 nt needs (i) at least three regions of at least six perfectly matching nt, (ii) two stretches of 12-15 perfectly matching nt, or (iii) one stretch of 15-18 perfectly matching nt to result in measurable hybridization. Probes with a high number of dInosines positioned at sites of variation need shorter matching regions than dInosine-free probes. It is suggested that this is probably because dInosine participates in nucleation and zipping during the hybridization process. Thus, a probe with 18 dInosines which match mismatches in the target needs either (i) two regions of 9 nt if the hybridization temperature is 55 C, or (ii) one region of nine perfectly matching nt at 45 C.