The authors have developed a genomic sequencing technique that can detect every methylated cytosine on both strands of any target sequence using DNA from fewer than 100 cells. The method involves treating DNA with sodium bisulphite to convert unmethylated cytosines to uracils while leaving 5-methylcytosines intact. The converted DNA is then amplified using specific primers and sequenced. This approach maximizes the efficiency of denaturation, bisulphite conversion, and amplification, allowing for the methylation mapping of single genes from small amounts of genomic DNA. The method is highly sensitive, with the ability to analyze methylation patterns in at least 20 gene sequences from DNA isolated from less than 100 cells. The non-destructive nature of the bisulphite conversion reaction also allows for the cloning and sequencing of PCR products, providing detailed information on methylation patterns in mixed populations of cells or DNA strands of different parental origin. The technique has significant implications for studying methylation in developmental processes, inherited diseases, and cancer.The authors have developed a genomic sequencing technique that can detect every methylated cytosine on both strands of any target sequence using DNA from fewer than 100 cells. The method involves treating DNA with sodium bisulphite to convert unmethylated cytosines to uracils while leaving 5-methylcytosines intact. The converted DNA is then amplified using specific primers and sequenced. This approach maximizes the efficiency of denaturation, bisulphite conversion, and amplification, allowing for the methylation mapping of single genes from small amounts of genomic DNA. The method is highly sensitive, with the ability to analyze methylation patterns in at least 20 gene sequences from DNA isolated from less than 100 cells. The non-destructive nature of the bisulphite conversion reaction also allows for the cloning and sequencing of PCR products, providing detailed information on methylation patterns in mixed populations of cells or DNA strands of different parental origin. The technique has significant implications for studying methylation in developmental processes, inherited diseases, and cancer.