Klinefelter syndrome is a sex chromosome disorder that occurs in approximately 1 in 600 males and is classically characterized by a 47,XXY karyotype. Patients generally present in adolescence or adulthood with gynecomastia, learning difficulties, developmental delay, hypergonadtropic hypogonadism, infertility, tall stature, and behavioral problems. Mosaic Klinefelter syndrome, found in roughly 10% of cases, is typically associated with a 47,XXY/46,XY karyotype; however, there are a few (<20) reported cases of 47,XXY/46,XX karyotype. These individuals often possess both testicular and ovarian tissues that can occur in the same gonad or separately. Here, we report on a 20-month-old (assigned male at birth) who presented to neurology with motor delay and hypotonia. Clinic notes mentioned an earlier diagnosis of mosaic Klinefelter syndrome, although no records were available for review. Cytogenetic and microarray analysis in our laboratory revealed a mosaic karyotype with 47,XXY in 15 cells and 46,XX in 5 cells. Microarray and FISH studies confirmed 82% 47,XXY and 18% 46,XX. Endocrine studies are consistent with testicular gonadal tissue and hormone levels suggest normal gonadal function. Testosterone level and thyroid function were both normal for his age. A pelvic ultrasound identified normal-appearing testicles and no evidence of a uterus or fallopian tube structures. The patient will continue to be followed by neurology and endocrinology. Clinical management of patients with differences in sex development (DSDs) is challenging and given the rarity of this chromosomal finding and highly variable reported phenotype of patients, it is difficult to predict the clinical phenotype for this patient. This case highlights the complexity in those presenting with DSDs and the need for further studies on individuals with similar findings.

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DOI: https://doi.org/10.1016/j.cancergen.2022.05.037

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