The heat-stable toxin I gene in the human Escherichia coli isolate 18D is the estA1 allele. The gene is not part of a composite transposon, but inspection of the flanking DNA sequences suggests that it was at one time part of a transposon. The hypothetical transposon originated from an event other than the occurrence that formed Tn1681.
Department of Molecular Genetics and Microbiology, Wellcome Research Laboratories, Burroughs Wellcome Co., Research Triangle Park, North Carolina 27709.
