To examine the specificity and the ability of the array to detect different isolates within subtypes of HIV-1 and different isolates of HIV-2 by the array, we additionally tested our primers with another panel, developed by the Southern Research Institute. This panel contained three different isolates from each subtype (A to G) of HIV-1 group M, three isolates from HIV-1 group O, one isolate from HIV-1 group N and three different isolates of HIV-2. The infectious dose of each HIV isolate in the panel was determined by TCID 50 (median tissue culture infective dose) titration and the dose of virus used in each PCR reaction was calculated. The results of testing of HIV-1 specific primers are shown in Figure 1 . In this experiment we evaluated the coverage of HIV-1 variants and estimated the relative sensitivity of the four HIV-1 specific primer sets based on cycle threshold (C(t)) values obtained with each isolate tested. Two primer sets targeting the conserved pol regions (NP170/171 and NP175/174) were able to detect most of the HIV-1 subtypes with a high sensitivity (C(t) = 15-25). Only one primer set (NP175/174) detected both the group N isolate and all three isolates of group O with Ct = 20-35. We did not detect crossreactivity of HIV-1 or HIV-2 specific primers with the other type of HIV. All three HIV-2 isolates studied (1-3 log 10 TCID 50 /PCR input) were detected with all HIV-2 specific primer sets with a low Ct value: 12-20 (data not shown).