Transcription factor that plays a role in early brain development. May activate transcription by interacting directly with the X-box. May activate transcription from CX3CL1 promoter through the X-box during brain development. Homodimer. Heterodimer with RFX2 and RFX3 (PubMed:11682486). Interacts with GPS2 (By similarity). Isoform 1: Expressed in brain and gliomas (at protein level). Isoform 2: Testis-specific (at protein level). Isoform 3: Testis-specific (at protein level). Isoform 3: Expressed at a higher level in adult testes and ejaculated spermatozoa than in fetal testes. Isoform 4: Testis-specific. Belongs to the RFX family. Probable intron retention. Truncated N-terminus. Truncated N-terminus. Truncated N-terminus.