Clone 33D3 recognizes the modified base 5-methylcytidine (5-MeCyd) found in plant and vertebrate DNA. It specifically distinguishes 5-MeCyd from its normal DNA base counterpart, cytosine, making it an ideal tool for gene promoter methylation analysis. DNA methylation is a postreplication process involved in the establishment of genomic imprinting, in the control of gene expression and of differentiation. Carcinogenesis is associated with alterations of the DNA methylation pattern: a global DNA hypomethylation is often detected in tumor tissues, associated with local hypermethylation sites. This antibody has been used to detect alterations in the urinary excretion of nucleosides by cancer patients, to visualize the distribution of methyl-rich regions along human chromosomes, to quantify in situ differences between normal and malignant cells from peripheral blood as well as on tissue sections. The antibody is ideal for the histopathological analysis of tissue samples without employing DNA extraction procedures that destroy cells.