cytosine arabinoside combines a cytosine base with an arabinose sugar. it is an antimetabolic agent with the chemical name of "1β-arabinofuranosylcytosine". certain sponges, where it was originally found, use arabinoside sugars to form a different compound (not part of dna). cytosine arabinoside is similar enough to human deoxycytosine to be incorporated into human dna, but different enough that it kills the cell. cytosine arabinoside interferes with the synthesis of dna. its mode of action is due to its rapid conversion into cytosine arabinoside triphosphate, which damages dna when the cell cycle holds in the s phase (synthesis of dna). rapidly dividing cells, which require dna replication for mitosis, are therefore most affected. cytosine arabinoside also inhibits both dna and rna polymerases and nucleotide reductase enzymes needed for dna synthesis. cytarabine is the first of a series of cancer drugs that altered the sugar component of nucleosides. other cancer drugs modify the base.