Before the late 20th century, research on progeria yielded very little information about the syndrome. In 2003, the cause of progeria was discovered to be a point mutation in position 1824 of the "LMNA" gene, which replaces a cytosine with thymine. This mutation creates a 5' cryptic splice site within exon 11, resulting in a shorter than normal mRNA transcript. When this shorter mRNA is translated into protein, it produces an abnormal variant of the prelamin A protein, referred to as progerin. Progerin's farnesyl group cannot be removed because the ZMPSTE24 cleavage site is lacking from progerin, so the abnormal protein is permanently attached to the nuclear rim. One result is that the nuclear lamina does not provide the nuclear envelope with enough structural support, causing it to take on an abnormal shape. Since the support that the nuclear lamina normally provides is necessary for the organizing of chromatin during mitosis, weakening of the nuclear lamina limits the ability of the cell to divide. However, defective cell division is unlikely to be the main defect leading to progeria, particularly because children develop normally without any signs of disease until about one year of age. Farnesylated prelamin A variants also leads to defective DNA repair, which may play a role in the development of progeria. Progerin expression also leads to defects in the establishment of fibroblast cell polarity, which is also seen in physiological aging.