1. School of Molecular & Cell Biology, Faculty of Science, University of the Witwatersrand, Johannesburg, South Africa
2. Current address: Division of Human Genetics, Faculty of Health Science, University of Cape Town, South Africa
3. Division of Human Genetics, Faculty of Health Sciences, School of Pathology, National Health Laboratory Service & University of the Witwatersrand, Johannesburg, South Africa
4. MRC/WITS Developmental Pathways for Health Research Unit, Department of Paediatrics & Child Health, Faculty of Health Sciences, University of the Witwatersrand, Johannesburg, South Africa