To investigate the detection range of the microarray on clinical samples, we performed analysis on urine samples positive for the circular virus BKV (Table 3 and Figure 4 ). Samples ranging from 15,810 to 4.0610 8 copies/ml of BKV were amplified by Phi29 polymerase for 16 hours using Repli-g Midi kit (Qiagen). Also included were three dilutions of a BKV urine sample, containing 892, 119 or 73 copies/ml, respectively. Samples containing $1000 copies/ml of BKV (BKV1-BKV5) were efficiently amplified with yields from 1.0610 11 to 1.9610 8 copies/ml of BKV (Table 3 and Figure 4A ), and with ,25 mg of DNA generated after amplification (data not shown). The total input of viral copies per 50 ml Phi29-reaction for BKV1-BKV5, were 2610 6 , 4100, 2100, 80 and 5 genomic copies, respectively ( Table 3 ). Amplification of samples containing ,100 copies/ml (BKV6 and BKV7) resulted in loss of reproducibility and low yields (Table 3 and Figure 4A ). The total input of viral copies per Phi29-reaction for these two samples were 0.6 and 0.4, respectively (Table 3) . Theoretically, amplification of samples containing 200 copies/ml means that ,1 viral copy is added to the Phi29-amplification reaction, resulting in stochastic problems due to unequal distribution in highly diluted solutions. Four amplifications were selected for microarray analysis (BKV3, BKV4, BKV5 and BKV6) and hybridized to the LLMDA (Table 3 and Figure 4B ), and analysed using the data analysis method developed at SSI. BKV + urine samples containing $1000 copies/ ml were clearly detected by the microarray analysis after Phi29amplification ( Figure 4B ). The human genomic sequence SSX3 (synovial sarcoma, X breakpoint 3) was used as a hybridization control and had the same signal intensity in all 4 microarrays. Some cross-hybridization with JCV was seen in the 3 samples where BKV was detected, even if they all were negative in a JCVspecific PCR (data not shown). This is similar to the BKV sample presented in table 2. From this we conclude that, from as little as 5 copies input ($1000 copies/ml) of BKV genomes the Phi29amplification was able to generate enough material for a clear detection by microarray analysis.