xlinked agammaglobulinaemia and the underlying genetics in two kindreds objective molecular analysis of the brutons tyrosine kinase btk gene in two unrelated families with a combined total of seven boys affected by xlinked agammaglobulinaemia xlamethods protein electrophoresis and western blotting were used for the examination of btk protein synthesis in blood leucocytes isolation of the coding sequence of the btk gene was performed by amplification using the reverse transcriptionpolymerase chain reaction rtpcr technique sequence alterations were screened for by the singlestranded conformation polymorphism sscp method and characterized by standard sequencing protocolsresults western blotting revealed btk protein to be absent in leucocytes of affected males from both families a novel 3 bp deletion in exon 3 of the btk gene was found to be responsible for the xla phenotype in the affected proband in one family kindred i a diagnostic pcr assay was established to detect this mutation in other affected male siblings and carrier females for the second family kindred ii the coding sequence of the btk gene and the promoter region were found to be normalconclusions the present study has demonstrated genetic heterogeneity in the btk gene in south african xla patients and has identified a novel mutation in this gene in the largest of the affected kindreds the gene mutation in the second kindred was undetermined and may be indicative of a defect in some other gene associated with btk function or stability western blotting was found to be informative in establishing a deficiency of btk protein in both probands and is recommended as a frontline procedure in the molecular diagnosis and workup of xla