Abstract
The childhood-onset SMA locus has been mapped to chromosome 5q13, in a region bounded by the proximal locus, D5S6, and the closely linked distal loci, DSS112 and MAP1B. We now describe a highly polymorphic, tightly linked microsatellite marker (D5S435) that is very likely the closest proximal marker to the SMA locus. Multipoint linkage analysis firmly establishes the following order of markers at 5q13: centromere-D5S76-D5S6-D5S435-MAP1B/D5S112-D5S39-telomere. The data indicate that SMA resides in an approximately 0.7-cM (range 0.1-2.1) region between D5S435 and MAP1B. This finding reduces by approximately fourfold the genetic region that most likely harbors the SMA locus and will facilitate the physical mapping and cloning of the disease gene region.