(+)-Yatakemycin (1, Fig. 1) and (+)-duocarmycin SA (2) are exceptionally potent, naturally occurring antitumor agents that derive their biological properties through a characteristic sequence-selective DNA-alkylation reaction. Studies have shown that both the AT-rich binding selectivity (shape-selective recognition) and the alkylation catalysis (shape-dependent catalysis) that contribute to the alkylation selectivity are dependent on the DNA minor groove shape and size characteristics of an AT-rich sequence (ref. 6 and references therein; refs. 7,8). Here we report the alkylation properties of yatakemycin and duocarmycin SA on free DNA (alpha-satellite DNA) and the same sequence bound in a nucleosome core particle (NCP) modeling the state of DNA in eukaryotic cells. Both compounds showed a clear, relatively unaltered ability to alkylate DNA packaged in NCPs in terms of both alkylating efficiency and sequence selectivity, despite the steric and conformational perturbations imposed by NCP packaging. These findings highlight the dynamic nature of NCP-bound DNA and illustrate that cell- and protein-free DNA-alkylation studies of members of this class of antitumor drugs provide valuable insights into their properties.