The developing prosess of transformation 1928, Fred griffith, working with pneumococcus(肺炎链球菌) avirulent strains could be restored to virulence by incubation with an extract from killed virulent cells 1944, Avery, macleod and mc carty demonstrated that the transforming principle waS DNA 1970, Mandel and hlga did the transformation by competent cells (chemical method, CaCl, deal with cells Now, transformation is very important because of its key role in gene cloning7 The developing prosess of transformation 1928，Fred Griffith, working with pneumococcus(肺炎链球菌) avirulent strains could be restored to virulence by incubation with an extract from killed virulent cells. 1944，Avery, MacLeod and McCarty demonstrated that the ‘transforming principle’ was DNA. 1970，Mandel and Hlga did the transformation by competent cells （chemical method，CaCl，deal with cells ） Now， transformation is very important because of its key role in gene cloning