Two patients with leukocyte adhesion deficiency (LAD), one with a moderate phenotype (patient 14) and one with a severe phenotype (patient 2) who had been shown to have a normal sized β subunit protein precursor, were analyzed in an attempt to determine the molecular basis for their disease. RNase mapping located possible mutations to two distinct but adjacent regions of the β subunit cDNA. Sequencing of patient-derived cDNA clones in this region revealed a C for T difference at amino acid 149 in patient 14 which resulted in the substitution of a leucine for a proline, and an A for G substitution at amino acid 169 in patient 2 which mutated a glycine to an arginine. The mutated amino acids are in a region of the cDNA that is highly conserved between the β subunits of the integrin family and are identical in all known integrin α subunits. Co-transfection of the a subunit cDNA containing the patient 2 mutation with the wild-type β subunit of LFA-1 in a mammalian expression system resulted in no expression of LFA-1. In the case of the mutation in patient 14 there was markedly diminished expression of LFA-1 with loss of function and loss of the epitope for a number of anti-β mAbs. Normal half-life of the mutant β subunits, and previous demonstration of a lack of α/β complex formation during biosynthesis in patient cells, suggest a defect in association with the a subunit. Association with 0 is required for expression of the a subunit of LFA-1. Loss of functional expression with both of these 0 subunit mutations suggests that they lie in a site critical for association with the a subunit.