Total RNA was prepared directly from the frozen pellets of parasitized erythrocytes, where approximately 1 ml of cell pellet was lysed in 7.5 ml Trizol (GIBCO) and RNA was extracted according to the manufacturer's instructions. mRNA was isolated from total RNA preparations using the Oligotex mRNA Mini Kit (Qiagen, Valencia, CA). For the hybridization experiments, 12 μg total RNA was used for first-strand cDNA synthesis as follows: RNA was mixed with a mixture of random hexamer (pdN6) oligonucelotides and oligo-(dT20) at final concentration 125 μg/μl for each oligonucleotide. The mixture was heated to 70°C for 10 min and then incubated on ice for 10 min. Reverse transcription was started by adding dNTPs to a final concentration of 1 mM dATP and 500 μM each: dCTP, dGTP, dTTP and 5-(3-aminoallyl)-2'-deoxyuridine-5'-triophosphate, (aa-dUTP) (Sigma), with 150 units of StrataScript (Stratagene, La Jolla, CA). The reaction was carried out at 42°C for 120 min and the residual RNA was hydrolyzed with 0.1 mM EDTA and 0.2 M NaOH at 65°C for 15 min. The resulting aa-dUTP-containing cDNA was coupled to CyScribe Cy3 or Cy5 (Amersham, Piscataway, NJ) monofunctional dye in the presence of 0.1 M NaHCO3 pH 9.0. Coupling reactions were incubated for a minimum of 1 h at room temperature. The labeled product was purified using QIAquick PCR purification system (Qiagen). Hybridizations and final washing procedures were carried out as described [9 (link)] with slight modifications. Briefly, the hybridization medium contained 3 × SSC, 1.5 μg/μl poly(A) DNA (Pharmacia Biotech, Uppsala), and 0.5% SDS. Hybridizations were incubated at 65°C for 8-16 h. Arrays were washed in 2 × SSC/0.2% SDS and then 0.1 × SSC at room temperature. The microarrays were scanned with a GenePix 4000B scanner and the images analyzed using GenePix Pro 3.0 software (Axon Instruments, Union City, CA). Subsequently, the data were normalized using the AMAD microarray database and subjected to the cluster analysis using the CLUSTER and TREEVIEW software, as described [53 (link)]. For the CLUSTER analysis, low-quality features and features with a signal level less than fivefold the background were filtered from the initial raw data set, yielding 4,737 elements. Subsequently, features with an arbitrary twofold fluorescence signal difference in at least four experiments were considered. All programs and microarray-related protocols are available online [55 ].