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Correspondence: Dr Germaine M. Buck Louis, Division of Epidemiology, Statistics & Prevention Research, Eunice Kennedy Shriver National Institute of Child Health & Human Development, NIH, 6100 Executive Boulevard, Rockville, MD 20852, USA. louisg@mail.nih.gov
The authors acknowledge the support of the Texas Parks and Wildlife Department in providing the database for study purposes.
The authors have no competing interests.
This study was supported by the Intramural Research Program of the Eunice Kennedy Shriver National Institute of Child Health and Human Development (contracts #N01-HD-3-3355, N01-HD-3-3356, N01-HD-3-3358).
© 2011 Blackwell Publishing Ltd.