We investigated DNA copy number aberrations in 37 cell lines derived from multiple myelomas (MMs) using comparative genomic hybridization, and 11 (29.7%) showed high-level gain indicative of gene amplification at 1q12-q22. A corresponding transcriptional mapping using oligonucleotide arrays extracted three up-regulated genes ( IRTA2 , PDZK1 , and S100A6 ) within the smallest region of overlapping in amplifications. Among them PDZK1 showed amplification and consequent overexpression in the MM cell lines. Amplification of PDZK1 was observed in primary cases of MM as well. MM cell lines with amplification of PDZK1 exhibited the resistance to melphalan-, cis- platin-, and vincristin-induced cell death compared with MM cell lines without its amplifications. Furthermore, down-regulation of PDZK1 with an anti-sense oligonucleotide sensitized a cell line KMS-11 to melphalan, cis- platin, and vincristin. Taken together, our results indicate that PDZK1 is likely to be one of targets for 1q12-q22 amplification in MM and may be associated with the malignant phenotype, including drug resistance, in this type of tumor.