Cell-cycle-stage specificity of the methotrexate block as resolved by X-ray-induced chromosome damage

Radiation-induced chromosome aberrations were used as biomarkers to compare G₀, mid-G₁, and methotrexate (MTX)-arrested lymphocytes. The ratio of chromosome-type to chromatid-type aberrations in MTX-arrested cells was consistent with that predicted when postreplicative chromosomes are exposed to ionizing radiation and supports the premise that MTX arrests cells in late S/G₂ of the cell cycle.