A histone, macroH2A, nearly three times the size of conventional H2A histone, was found in rat liver nucleosomes. Its N-terminal third is 64 percent identical to a full-length mouse H2A. However, it also contains a large nonhistone region. This region has a segment that resembles a leucine zipper, a structure known to be involved in dimerization of some transcription factors. Nucleosomes containing macroH2A may have novel functions, possibly involving interactions with other nuclear proteins.