The explanation for this phenomenon is that male carriers pass on their premutation to all of their daughters, with the length of the "FMR1" CGG repeat typically not increasing during meiosis, the cell division that is required to produce sperm. Incidentally, males with a full mutation only pass on premutations to their daughters. However, females with a full mutation are able to pass this full mutation on, so theoretically there is a 50% chance that a child will be affected. In addition, the length of the CGG repeat frequently does increase during meiosis in female premutation carriers due to instability and so, depending on the length of their premutation, they may pass on a full mutation to their children who will then be affected. Repeat expansion is considered to be a consequence of strand slippage either during DNA replication or DNA repair synthesis.