In the strategy of DNA synthesis without base protection, a recent O-selective phosphorylation procedure known as the "activated phosphite method" is used to generate reactive phosphite intermediates from N-unprotected phosphoramidite building blocks. This new strategy proved to be advantageous over previous strategies because the cleavage of the N-P(III) bond, which can be formed by the significant phosphitylation of the nucleobase amino groups, can be omitted. The activated phosphite method provides an attractive route to high-throughput synthesis of DNA and the synthesis of alkali-labile modified DNAs such as oligonucleotides containing DNA lesions. This unit presents two convenient methods for the synthesis of N-unprotected deoxynucleoside 3'-phosphoramidite units and detailed procedures for automated synthesis of oligodeoxyribonucleotides from these phosphoramidite units.