Phenylbutyrate acts on multiple targets to sensitize head and neck cancer cells to cisplatin. Top panel: BRCA1-defective UM-SSC-17B cells  were mock treated for 5 days (control), treated with 2 mM phenylbutyrate for 5 days (phenylbutyrate), treted with 2 μM cisplatin for 3 days (cisplatin) or treated with 2 mM phenylbutyrate for 5 days and 2 μM cisplatin for the last 3 days (PB+cis). At the completion of the treatment, both floating and attached cells were collected and analyzed by flow cytometry. Middle panel: same as above but using the FA/BRCA1-defective UM-SSC-14A cells . Lower panel: Same as above but using the FA/BRCA1-proficient cell line UM-SSC-6 . Note that the FA/BRCA1-defective cells appear to struggle through late S-phase as evidenced by gradual accumulation of cells in later stages of S-phase while the proficient UM-SSC-6 cell line do not show this accumulation. The percentages of sub-G1 DNA-containing cells (apoptotic cells) are presented in each panel.