Spinal muscular atrophy (SMA) is the second most common lethal, autosomal recessive disease In Caucasians, second only to cystic fibrosis. in an effort to identify the causative gene in SMA, we have used radiation hybrid (RH) mapping to prepare a high resolution physical map of the proximal region of chromosome 5 (5q11–13) which contains the SMA gene. The map of the SMA region, which spans -4 Mb, contains 19 loci Including 9 polymorphic DNA markers, 8 monomorphic sequence tagged sites (STS) and two genes. Based upon the RH map the two polymorphic loci which most closely flank the SMA locus were estimated to be separated by ?750 kb. Using two different directional cloning schemes, several new clones between the genetic markers which most closely flank SMA were Isolated. These new clones within the SMA candidate region, together with cosmid clones prepared from one RH hybrid which retains an ?1 Mb segment spanning the SMA region as its only human DNA, will greatly facilitate efforts to Identity the gene for SMA. in addition, analysis of cloned DNA segments from within the SMA candidate region has Identified the presence of a novel, chromosome 5-specific, low copy repeated sequence which Is distributed throughout the region containing the SMA gene as well as in at least four other regions of chromosome 5. Whether or not these novel repeated sequences throughout the SMA region are Involved in the disease remains to be determined.