KIT and atrial fibrillation: This is probably due to the observation that, comparing cells obtained in the second trimester but at early (E) or late (L) weeks of pregnancy, the percentage of c-kit positive cells was 5 times higher in cultured E-AF, compared to cultured L-AF [18], indicating that this subpopulation (c-kit positive) is destined to decrease during the gestation, thus it would be not useful to isolate a so scarce subpopulation.