In mammals the methylation of cytosine residues in DNA by DNA methyltransferases (DNMTs) is the predominant postreplication base modification (Das and Singal 2004). Mammalian DNMTs transfer a methyl group from S-adenosylmethionine (SAM) to the C-5 position of cytosine within CpG dinucleotides (Das and Singal 2004). DNMTs found in mammalian cells include DNMT1, DNMT3a, and DNMT3b (Okano et al. 1998, 1999a). It is thought that DNMTs function in either maintenance of DNA methylation, where methylated CpG sites on one DNA strand are copied, or in de novo methylation, where both strands are initially unmethylated and methylation at novel sites is introduced (Das and Singal 2004). Specific forms of DNMT include DNMT1, which is considered important for maintenance and is constitutively expressed in adult cells, and DNMT3a and 3b, which have strong de novo activity (Das and Singal 2004) and are highly expressed in embryonic cells (Okano et al. 1999b), presumably to assist in methylation changes associated with development. Defects in methylation of total DNA or particular DNA sequences, including hypomethylation and, conversely, hypermethylation, have been shown to be associated with carcinogenesis, possibly as a factor that facilitates aberrant under- or over-expression of genes linked to cancer (Das and Singal 2004). Aberrant methylation of promoter regions resulting in inactivation of human tumor suppressor gene expression has been proposed to be an important mechanism in cancer progression (Baylin et al. 1991). Understanding what modulates changes in DNA methylation during malignant transformation is a key issue in chemical carcinogenesis. In this regard, during carcinogenesis, global DNA hypomethylation together with gene-specific hypermethylation, often, but not always, occur. It is suspected that DNMT1 underexpression causes the total DNA hypomethylation, whereas over-expression of DNMT3a or 3b induces gene-specific promoter region hypermethylation and quiescence of tumor suppressor genes (Baylin et al. 1991; Chen et al. 2003). For example, hypermethylation in the promoter region and silencing of p16 tumor suppressor gene are frequently observed during carcinogenesis that can occur on a background of global DNA hypomethylation (Wu et al. 2005). Cancer cells may be distinctive in that DNMT1 alone is not responsible for maintaining abnormal gene-specific hypermethylation, and both DNMTs 1 and 3b may cooperate in this action (Rhee et al. 2000, 2002). However, the precise events that account for changes in global or gene-specific methylation patterns in carcinogenesis remain uncertain. Methylation changes occur during oncogenic transformation with inorganic carcinogens such as arsenic, nickel, and cadmium (Benbrahim-Tallaa et al. 2005; Salnikov and Costa 2000; Takiguchi et al. 2003). For cadmium, a heavy metal classified as a human carcinogen, human exposure is associated with lung and possibly prostate cancer, whereas in rodents the metal is clearly a prostate carcinogen (Waalkes 2003). In in vitro model systems of carcinogenesis, cadmium exposure malignantly transforms various human and rodent cells, which give rise to aggressive cancers when injected into mice (Achanzar et al. 2001; Qu et al. 2005; Waalkes 2003). In particular, cadmium induces malignant transformation of the normal human prostate epithelial cell line RWPE-1 (Achanzar et al. 2001), which supports the potential for cadmium to directly target this cell population in vivo. In this prior work (Achanzar et al. 2001), to achieve malignant transformation, control human prostate epithelial RWPE-1 cells were exposed continuously to 10 μM cadmium for 8 or more weeks, a concentration near the estimated range in the prostate of adult human males with no known occupational exposure to cadmium. The cadmium-treated cells, designated CTPE (cadmium-transformed prostate epithelial cells), form malignant tumors after inoculation into nude mice that morphologically and biochemically resemble human prostatic carcinoma (Achanzar et al. 2001). Thus, the cadmium-transformed CTPE cells provide a human model system in which to examine the molecular events during cadmium carcinogenesis in the prostate. Previous work showed an unusual pattern of enhanced generalized DNMT activity and global DNA hypermethylation during cadmium induction of malignant transformation in rat liver epithelial cells (Takiguchi et al. 2003). Thus, in the present study, we assessed the impact of chronic cadmium exposure leading to acquisition of a malignant phenotype on DNA methylation and DNMT activity in a human cell model using CTPE cells.