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Filaggrin ( FLG) plays an important role in the barrier function of the skin. Several loss-of-function mutations in the FLG gene have been identified in patients with ichthyosis vulgaris, and these null mutations are associated with atopic dermatitis (AD) development. In this study, we examined tag single nucleotide polymorphisms (tSNPs) and null mutations in FLG for possible associations with AD and atopic phenotypes in a Japanese population. Transmission disequilibrium test of 105 AD families showed that the null allele of the S2554X variant of FLG tended to be overtransmitted to AD-affected offspring; however, the P value did not reach statistical significance. In a case–control comparison of 376 AD cases and 923 nonallergic controls, the null allele of S2554X was significantly associated with AD ( P = 0.0012), and the association was strengthened in subjects with AD alone ( P = 0.000024). We found that 3321delA and S2554X were also associated with elevated levels of immunoglobulin E (IgE). Combined null mutation carriers were observed more in AD patients and in subjects with high IgE than in control subjects. The combined P value for the family and case–control data was significant for the S2554X and combined null mutations. Our data further support the importance of FLG in AD development. [ABSTRACT FROM AUTHOR]