Steroid receptor complexes bind to specific nuclear acceptor sites on chromatin to induce alterations in gene expression. This paper describes many factors regulating the nuclear binding of the avian oviduct progesterone receptor. Some of these factors occur in vivo and appear to be involved in natural regulatory processes, whereas other factors are detected only in cell-free in vitro binding assays and affect the amount of nonspecific binding. Such variations in nonspecific binding in vitro could cause misinterpretation of cell-free nuclear binding data and could lead to erroneous identification of nuclear acceptor sites for other steroid receptors in other animal tissues.
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