Introduction:
Sex chromosome-gender phenotype discordance is a rare phenomenon in humans and generally associated with conditions that cause some sort of gonadal impairment. We report on the clinical, laboratorial and genetic studies of a phenotypically-normal woman with 46,XY karyotype and completely normal gonadal function.

Methods:
Samples from blood, urine, buccal swab and skin biopsy from the female proband were analyzed through cytogenetic techniques, HLA typing and STR analysis and then the results were compared to her male twin brother in an attempt to clarify the etiology of her 46,XY karyotype.

Results:
The proband has had a completely normal life with no remarkable health events and a normal sexual development; she had an early pregnancy loss following a spontaneous pregnancy at the age of 35. Karyotype in two repeated blood analyses resulted in 46,XY in all metaphases; FISH confirmed this pattern by showing one X and one Y signal in all 100 cells analyzed. Cytogenetic analyses of urine, buccal mucosa and fibroblasts resulted in 46,XX. STR profile and HLA genotyping of proband's blood was completely identical to her twin brother's, including the sexual markers, and divergent from her other tissue samples.

Conclusion:
The presence of different cell populations present in different tissues suggests that the proband is a chimera. Cytomictical chimerism, also known as blood chimeras or twin chimeras, is a rare cause of sex chromosome-gender phenotype discordance and may manifest without any sexual anomalies or reproductive impact.