Genetic Basis of Hearing Loss 
netic maps composed of very polymorphic, ho- 
mogeneously distributed PCR-based markers. 
Genetic mapping can localize a disease gene to 
a 1- to 2-Mb region of a particular chromosome. 
The next challenge is to identify the genes in this 
region. We have developed a strategy, termed 
exon trapping, that utilizes splicing signals as 
primary identifiers of coding sequence in cloned 
genomic DNA. This approach is also of great util- 
ity for cloning the human homologue of a gene of 
interest when the temporal or spatial pattern 
of gene expression limits the availability of 
mRNA for analysis. We are developing second- 
generation exon-trapping strategies based on 
large-capacity cloning vectors (e.g., cosmids, 
PI ) that should extend the range for gene search- 
ing and help to integrate the recovery of genes 
with physical mapping. 
120 
