Lamin A does not appear to be necessary for life; mice in which the "Lmna" gene is knocked out show no embryological symptoms (they develop an Emery–Dreifuss muscular dystrophy-like condition postnatally). This implies that it is the buildup of prelamin A in the wrong place, rather than the loss of the normal function of lamin A, that causes the disease. It was hypothesized that part of the reason that treatment with an FTI such as alendronate is inefficient is due to prenylation by geranylgeranyltransferase. Since statins inhibit geranylgeranyltransferase, the combination of an FTI and statins was tried, and markedly improved "the aging-like phenotypes of mice deficient in the metalloproteinase Zmpste24, including growth retardation, loss of weight, lipodystrophy, hair loss, and bone defects". Repair of DNA double-strand breaks can occur by either of two processes, non-homologous end joining (NHEJ) or homologous recombination (HR). A-type lamins promote genetic stability by maintaining levels of proteins that have key roles in NHEJ and HR. Mouse cells deficient for maturation of prelamin A show increased DNA damage and chromosome aberrations and have increased sensitivity to DNA damaging agents. In progeria, the inability to adequately repair DNA damages due to defective A-type lamin may cause aspects of premature aging (also see DNA damage theory of aging). Fibroblast samples from children with Hutchinson–Gilford progeria syndrome exhibit accelerated epigenetic aging effects according to the epigenetic clock for skin & blood samples .