Underlining the vulnerability of retinoblastoma cells to pyrimidine nucleotide synthesis inhibitors, we have shown that loss of chr16q22.2 (containing the DHODH gene) and gains in chromosome regions expressing pyrimidine ribonucleotide salvage factors (uridine transporter ENT1 and two uridine kinases) are frequent in metastatic retinoblastoma. The gene discussed is UCK2; the disease is retinoblastoma.