The purpose of this study was to develop and evaluate rapid, accurate and convenient methods to be used in the diagnostic field of medicine. To achieve this aim, fluorescence in situ hybridisation (FISH) methods were used on breast cancer patients. Whole chromosome paints (WCPs) proved positive for only three samples. Various abnormalities were identified, including a translocation of breakage of chromosome 17 in one patient. The remaining two patients demonstrated random translocations of chromosome 1 onto other autosomal chromosomes. These patients had undergone chemotherapy (CMF), it is possible, such treatment could cause these aberrations. The p53 probe revealed two patients with a deletion of the p53 gene with significant value <0.05. The methodology was evaluated by studying Charcot-Marie-Tooth disease which is suspected to have a duplication of p53 gene, results revealed a significant amplification of the gene. The need for an efficient and cost effective screening technique has led to the use of PCR-SSCP to detect mutations in breast cancer patients. SSCP detected 9/81 mutations. These were then, characterised by direct sequencing. Transversion mutations were the most frequently detected which is consistent with sporadic breast cancer. In addition, a transition mutation, a base insertion and a deletion were detected. Comparative Genomic Hybridisation (CGH) was used to screen the whole genome of blood and tumour tissues of breast cancer patients. This new technique has revealed amplifications and deletions of some genes which are associated with advanced stages of the disease. Finally, CGH was applied on DNA of Retts syndrome patients. The results highlighted two regions on chromosome X (Xq28 and Xp22.1-pter) involved in deletion or amplification of chromosome X. In addition, various autosomal chromosomes were also involved. This indicated that both X-linked and autosomal regulatory regions may be part of a complex alteration in association with Retts syndrome.