Abstract: The present invention relates to methods for decondensation and/or condensation of chromatin or chromosomes through the addition of substances to condensed or decondensed chromatin or chromosomes respectively. In particular, the invention relates to a method for decondensing and subsequently optionally recondensing chromatin in cells at the interphase stage of the cell cycle and to a method for condensing chromatin or chromosomes following disruption of sperm. The present invention also relates to substances for the decondensation and/or condensation of chromatin or chromosomes and to kits comprising said substances, as well as to the use of these substances in chromatin or chromosome analysis, reproduction in particular fertility assessment, the diagnosis and treatment of infertility, assisted conception either in the clinic or naturally and contraception. A particular factor identified is a decondensation factor which has a molecular weight of 20 kDa and which is obtainable from HeLa cells.