A considerable number of gene mutations has now been reported in a total of more than 1000 different human genes. Data on these mutations and their associated phenotypes have been collated and are available online through two major databases: Online Mendelian Inheritance in Man in Baltimore and the Human Gene Mutation Database in Cardiff. Since the non-randomness of mutation is determined largely by the local DNA sequence environment, the study of mutation may not only yield information on underlying mechanisms but also lead to the optimization of mutation search strategies. CONCLUSION: There is a high frequency of CG to TG or CA mutations in the human genome due to deamination of 5' methyl-cytosine. The second most common type of mutations in human disorders is short deletions or insertions of less than 20 nucleotides.