Detection of neo gene by PCR analysis of individual primary colonies after clonogenic assays. HFBM was transduced with LN, a retroviral vector containing the neo gene as described in Materials and Methods. The cells were then plated in clonogenic assays and after 2 weeks, random colonies were picked and assessed for incorporated neo gene by PCR. A representative ethidium bromide gel showing the presence of the neo primer pair-amplified product in randomly picked colonies is shown.