DNA exists as single molecules in individual cells.Point mutations and copy number variations, which are major dynamical changes of DNA, can now be studied at the single cell level by whole genome amplification and sequencing.Experiments probing the biology of meiosis and cancer will be described.We have demonstrated the pre-implantation genomic screening of embryos for in vitro fertilization in order to avoid miscarriage and genetic diseases.We also studied circulating tumor cells, which provide genetic signatures for personalized therapy of cancer.