A clinical study published in JCEM Case Reports describes two sisters born with atypical sex development whose origin has just been traced to a genetic mutation never described before. The case serves as a reminder that the biology of sex and gender identity are far more complex realities than the simple division into two categories suggests.
Both sisters were assigned female at birth, grew up and developed as females, and show no gender incongruence. The first consultation came, simply, because of primary amenorrhea: they had never had a menstrual period. The tests revealed something the external appearance did not betray: their karyotype was 46,XY, the one usually associated with male development.
Imaging showed female external genitalia with minimal virilization, absent Müllerian structures (the precursor of uterus and fallopian tubes), and testes located in the inguinal region. Hormonally, androgen synthesis was markedly reduced: very low testosterone and a high 17-hydroxyprogesterone-to-androstenedione ratio, the typical pattern of an isolated 17,20-lyase deficiency.
A novel mutation inside the gene
Whole-exome sequencing found the explanation in the CYB5A gene, which encodes cytochrome b5, a protein required for the 17,20-lyase enzyme to build testosterone precursors. The two sisters share a previously unknown homozygous variant (c.140G>T; p.Gly47Val) in exon 2 of the gene, and segregation analysis confirmed that both parents are healthy heterozygous carriers.
The finding expands the catalog of genetic causes of disorders of sex development (DSD), specifically of the so-called 46,XY DSD. The novelty of the variant and the fact that it appeared in two sisters strengthen its causal role: it is a recessively inherited mutation that interrupts an essential step in androgen production.
Methemoglobinemia: the clue nobody was looking for
The study also left a second clinical clue. When methemoglobin levels were measured, both cases were above the normal range despite the absence of symptoms. This is no coincidence: cytochrome b5 also participates in reducing methemoglobin, so its deficiency is associated with a silent elevation of this compound. The authors point out that, in a patient with DSD and suspected 17,20-lyase deficiency, methemoglobinemia should be actively looked for.
What it means for identity
The case holds special interest beyond the molecular diagnosis. Multidisciplinary evaluation confirmed female gender identity in both sisters, with no incongruence, and maintaining the female gender assignment in which they grew up was recommended. After the age of 18, gonadectomy and vaginoplasty were planned.
The story illustrates how anatomy, hormones and chromosomes do not by themselves determine a person’s identity. Here, a gene whose defect drastically reduces fetal testosterone coexists with a fully established female identity. It also underlines the relevance of genetics in understanding the diversity of human sex development and in the personalized care of those living with a DSD diagnosis.
Source: JCEM Case Reports. Published September 2026, volume 4, article luag299.
