In the period 1998-2000 almost all new cases of childhood spinal muscular atrophy (SMA) in addition to those from our database were studied for possible deletion of SMN gene (exons 7 a 8) and NAIP (exons 5 a 6). We correlated the size of deletion with the type, course and the onset of disease. The most informative for diagnosis was deletion of SMN. NAIP was deleted only in 18% of all cases, usually in SMA1 (in only 2 cases of SMA2). The molecular genetics permits to come back to previously posed question about the extremely frequent intrafamilial variability in SMA families. The problem became more clear due to our knowledge on telomeric and centromeric copies, as well as various genes and paragenes involved in SMA region. In the premolecular era we postulated the gender influence on course of the disease, which is particularly evident in mild SMA form 3b. Interestingly, presently we and some others detected deletion similar to those in the patients among the clinically healthy siblings. Those siblings are mostly females. The other problem, byproduct of molecular genetic, is the occurrence of SMN deletion in some disorders till now considered entirely different from SMA, e.g., arthrogryposis, congenital hypomyelinating neuropathies of newborn.