Risk allele burdens for familial MM, sporadic MM, and controls. Risk allele burdens for the 3 study groups, quantified as (A) number of risk alleles, and (B) sum of number of risk alleles per locus weighted by their log-transformed ORs. We detected an enrichment of MM risk alleles in familial cases compared with both sporadic cases and population-based controls. The indicated P values were obtained with 1-sided Student t test. Error bars indicate standard error of the mean. Of the 38 familial MM cases, 30 had a first-degree relative with MM and 8 had a second-degree relative. No further increase in average risk score was observed when excluding the cases with second-degree relatives (not shown).