Sex determination refers to the sexual characteristics of an organism, male or female, based on the chromosomal and genetic determinants. Sexual differentiation, is the physiologic and anatomic process that results in the development of a male or female phenotype. This process is directed by hormones, secreted by gonads, that influence the differentiation of external and internal genital structures. Therefore, sexual determination is contingent on a genetic regulation, in which SRY is not the only gene necessary for testicular differentiation (some other genes include SOX9) [1,2,3]; like so a gonadal and a hormonal regulation stage .
A genetic decontrol of genes such as SRY, SOX9, DAX-1, WNT4, WT1, DHH, and SF-1 can result in disorders of sex development (DSD), which refer to congenital conditions of atypical development of the chromosomal, gonadal or anatomical sex. During this stage, mutations, deletions or duplications in several of these genes have been identified, and affect the sexual differentiation and final phenotype of the individual. As in the case of gonadal dysgenesis in 46,XY individuals with DAX-1 duplication [4,5,6].
DAX-1 (dosage-sensitive sex reversal (DSS), adrenal hypoplasia congenital critical region on the X chromosome, gene 1), also known as NR0B1 (Nuclear Receptor Subfamily 0 Group B Member 1), is a member of superfamily of nuclear receptors. This gene is located in short arm of Xp21, contains 2 exons  and encodes a protein of 470 amino acids, a potential transcription factor expressed in the adrenal glands, but also in the hypothalamus, pituitary gonadotropic cells and gonads [8,9,10,11]. DAX-1 acts as a dominant-negative coregulatory protein that inhibits the transcriptional activity of other nuclear receptors SF-1 , estrogen receptor  and androgen receptor  mediated by the retinoic acid receptor [15, 16]. It has DNA-binding domain at the N terminus and the C terminus with characteristics of a nuclear hormone receptor ligand-binding domain.
The existence of a gene in the X-chromosome involved in human sex determination was reported by German et al.  assumed that Xp duplication was associate to sex reversal due to double dosage of an X-linked gene which is normally subject to X-inactivation, therefore this locus has previously named DSS . It has been postulated as an “anti-testis” gene based on the finding of XY patients with duplication in Xp21 with sexual reversion and dimorphic expression between ovary and testis [16, 18,19,20,21]. Hypotheses about the etiology indicate that excess of DAX-1 protein reduces activation of the SOX9 enhancer by inhibiting the interaction of SF1 on WT1, antagonizing steroidogenesis and the production of anti-Müllerian hormone (AMH), it was validated in transgenic mice by Ludbrook et al. . Although the molecular mechanisms by which SRY acts are not precisely known, there is experimental evidence that SRY and DAX-1, interact in early periods of development of the gonadal ridges, expressing both in testicular and ovarian tissue [23, 24]. The exactly time of expression and function of SRY and DAX-1 is important due to a delay in SRY expression that would allow an anti-testicular action of DAX-1, resulting in the formation of ovotestes or dysgenetic gonads . Disorder of sex development in the absence of DAX-1 occurred after normal expression of SRY, which evidence that both are required for normal testis determination.
Furthermore, point mutations and deletions in the DAX-1 gene cause congenital adrenal hypoplasia (AHC) and hypogonadotropic hypogonadism (HH) , secondary to not differentiation of adrenal gland beyond the fetal stage with disturbance in steroidogenesis and defective development of testis cords . Unlike duplications in DAX-1 in XY individuals undergo sex reversal and develop as females, XY transgene mice carrying extra copies of Dax-1 had delayed testis development and reduced expression of Sry and Sox9, suggesting a Dax1-Sox9 antagonism, but do not normally show sex reversal, the complete sex reversal occurred, when the transgene mice were tested against weak alleles of Sry gene . In the study of Ludbrook et al. in mouse model using reporter gene evidenced that Dax1 overexpression reduced activation of TES, the testis enhancer of Sox9, indicating that Dax1 might repress Sox9 expression via TES .
Several families of 46,XY individuals with female phenotype and gonadal finding of gonadal streaks or gonadoblastoma have been reported [8,9,10,11, 15, 17,18,19, 26, 27]; suspicion of “sex reversal” in families without parental consanguinity and the characteristic distribution of affected individuals suggest X-linked recessive inheritance. Bardoni et al. identified a region of approximately 20-Mb on Xp21.2–p22.1 that was duplicated only in the 46,XY females . The similarities in the phenotype of the 46,XY females with different Y chromosome (from different fathers) is the best evidence to explain that the Y is not enough to differentiation of the testis, and is the interaction between genes of X and Y chromosomes that defines gonadal differentiation.
46,XY sex reversal occur in about 1 in 3000 births , in 15% of the patients it is secondary to SRY deletions or mutations  in the other percentage of patients, mutations in other genes have been identified . We report the case of two sisters with duplication of the Xp chromosome segment within the region of Xp21.1–22.2 (DAX1) resulting in 46,XY sex reversal.