FLG and Harlequin ichthyosis: Considerable effort has been devoted to understanding genotype to phenotype correlations, including recent work showing deficiency in filaggrin (FLG) as a prime factor in ichthyosis vulgaris (IV) [10], STS deletions resulting in altered ultrastructure and gene expression [11], [12], defects in ATP-binding cassette subfamily A12 (ABCA12) responsible for harlequin ichthyosis [13], [14], and mutations in transglutaminase 1 (TGM1) causing approximately one-third of autosomal recessive congenital ichthyoses and the majority of the LI subtype [15], [16].