Two features of heterochromatin: heteropycnosis (high values of chromatin condensation), and repressed genetic expression, force upon us the use of cooperative variables, rather than molecular ones. In particular a "repressor" hypothesis is formulated, in which a useful parameter is clearly identified. This enables us to discuss the synchronized repression of a large number, n, of genes (as in the case of the Barr body, in which n is larger than 100). The hypothesis is documented with phenomena known to occur in active chromatin. Possible tests are suggested.