Abstract: A system for targeting dividing cells and delivering therapeutic agents to the targeted cells using a Tag nanoparticle. Each Tag nanoparticle comprises a nucleotide encoding a Tag, which is expressed only in dividing cells. The system optionally utilizes a microparticle to deliver Tag nanoparticles to cells. Therapeutic nanoparticles comprising therapeutic compounds are subsequently targeted to the tagged cells via surface molecules that specifically bind to the Tag. The system may be adapted for specific patient needs, for example, targeting and eliminating cancer cells, preventing metastasis and secondary tumors, stimulating or inhibiting proliferation of endothelial cells, and stimulating cell regeneration in diseased organs such as liver.