Frequency gradients of DHCR7 mutations in patients with Smith-Lemli-Opitz syndrome in Europe: evidence for different origins of common mutations. - Witsch-Baumgartner, M; Ciara, E; Löffler, J; Menzel, H J; Seedorf, U; Burn, J; Gillessen-Kaesbach, G; Hoffmann, G F; Fitzky, B U; Mundy, H; Clayton, P; Kelley, R I; et al. (European Journal of Human Genetics, 2001)