The region of the crossover causing the Filipino type of alpha thalassaemia has been determined by examining similarity between the regions which had been indicated as involved in the crossover points by restriction mapping, using the published alpha-globin region DNA sequence. The crossover point was found in 21 base pairs between two Alu sequences using PCR primers flanking these Alu sequences. A simple PCR multiplex assay has been devised to detect heterozygotes and homozygotes.