Purine de novo pyrimidine synthesis

In a patient receiving 6-mercaptopurine treatment for leukemia, which aspect of nucleotide synthesis is most directly affected?

1: Conversion of ribonucleotides to deoxyribonucleotides
2: Synthesis of purine nucleotides from PRPP
3: Conversion of dUMP to dTMP
4: Synthesis of pyrimidine nucleotides