R3H domain of the X1 box binding protein (NF-X1) and related proteins. Human NF-X1 is a transcription factor that regulates the expression of class II major histocompatibility complex (MHC) genes. The Drosophila homolog shuttle craft (STC) has been shown to be a DNA- or RNA-binding protein required for proper axon guidance in the central nervous system and, the yeast homolog FAP1 encodes a dosage suppressor of rapamycin toxicity. The name of the R3H domain comes from the characteristic spacing of the most conserved arginine and histidine residues. The function of the domain is predicted to bind ssDNA or ssRNA in a sequence-specific manner.