The use of RO2-0683 as differential inhibitor has shown that 13% of 181 heterozygotes, believed to be E1uE1a, are in fact E1aE1k. The results indicate that slightly more than 1% of the British population could be homozygote E1kE1k. Analysis of 47 families segregating the E1aE1k phenotype in more than 2 blood relatives is restricted by the inability to differentiate the genotypes E1uE1k and E1kE1k from E1uE1u and E1uE1f.