Transcription factor that acts as a key regulator of spermatogenesis (By similarity). Acts by regulating expression of genes required for the haploid phase during spermiogenesis, such as genes required for cilium assembly and function (By similarity). Recognizes and binds the X-box, a regulatory motif with DNA sequence 5'-GTNRCC(0-3N)RGYAAC-3' present on promoters (PubMed:14743396, PubMed:15526285, PubMed:16676351, PubMed:18247329). Probably activates transcription of the testis-specific histone gene H1-6 (PubMed:14743396, PubMed:15526285, PubMed:16676351, PubMed:18247329). Homodimer; probably only forms homodimers in testis. Heterodimer; heterodimerizes with RFX1 and RFX3. Mainly expressed in the nucleus and at lower level in cytoplasm. Expressed at highest level in testis. Expressed at lower level in thymus. Also expressed in stomach, kidney, liver, brain and heart. Weakly expressed in spleen and lung (PubMed:16676351). Within testis, most abundantly present in spermatocytes: present from pachytene spermatocytes to early spermatids (at protein level) (PubMed:15229132, PubMed:16676351). Also present in non-germinal tissues (PubMed:16676351). Belongs to the RFX family.