[en] The network structure of the entangled chromatin fibers in chromosome plays a key role in molecular control mechanism involved in chromosome mutation due to ionizing radiations or chemical mutagens. The network system of chromatin, namely 'chromatin network', is not so rigid and regular as has been postulated in general. We have proposed a rheological explanation for the flexible network system that consists of the fluctuating assembly of nucleosome clusters linked with supertwisting DNA in a chromatin fiber ('Supertwisting Particulate Model': Watanabe 1978). Applying the supertwisting particulate model, we have proposed a 'Heterosensitive Target Model' (Watanabe 1982) that is a modification of 'Heterogeneous Target Model' (Watanabe 1976). The heterogeneity of chromatin network is derived from the highly condensed organization of chromatin segments that consist of hypersensitive and fragile sites in the fluctuating assembly of nucleosome clusters 'superbeads'. The above consideration is going to be subjected to a new experimental approach applying the 'atomic force microscope (AFM)', one of the most promising members of a family of 'scanning probe microscope (SPM)'. Using the AFM with a liquid immersion cell, the isolated chromosomes were submerged in a solution of culture medium and manipulated on the nanometer scale. The particulate chromatin segments of nucleosome clusters (superbeads) were clearly observed within mitotic human chromosomes in a living hydrated condition. These findings support the heterogeneity of chromatin target in a living cell. (author)