Two partial human brain sodium channel cDNA sequences (designated HBSC I and II) have been cloned and mapped to chromosome 2q23–2q24 by chromosome microdissection-PCR (CMPCR). The distribution of HBSC I and II mRNA in human brain was studied by means of a novel approach based on the ligase detection reaction. These studies demonstrate that HBSC I and II mRNA is heterogeneously distributed in brain. and that the relative ratio or the two forms can vary as much as 7-fold between different regions.