A permanent lymphocyte cell line of a heterozygote with Yunnanese (A gamma delta beta)0-thalassemia deletion, associated with an increased production of G gamma globin in adult, was founded using Epstein-Barr virus transformation. The hybrids of the lymphocyte cell and mouse erythroleukemia cell (MEL) were achieved and the hybrids containing human chromosome 11 were selected with the monoclonal antibody 53/6. The subclones containing only either the normal or the abnormal human chromosome 11 were separated and the expression of the human globin genes was studied. Expression of the beta-globin gene, but not the G gamma and A gamma, was observed in the hybrids containing only the normal human chromosome 11, while active expression of the G gamma globin gene was observed in the hybrids containing only the abnormal human chromosome 11. These results have confirmed that the DNA deletion in the beta-globin gene cluster is the cause of persistent active expression of the G gamma globin gene in the Yunnanese mutant.