Abstract K. Bangalore Krishna: None. F.X. Schneck: None. M. Reyes-Mugica: None. S.A. Yatsenko: None. S.F. Witchel: None. Introduction: Individuals with 45,X/46,XY karyotypes have increased risks of developing germ cell tumors such as gonadoblastoma, defined as an in situ form of malignant germ cell tumor consisting of germ cell neoplasia in situ (GCNIS) / seminoma / dysgerminoma cells and incompletely differentiated sex cord cells reminiscent of Sertoli/granulosa cells. Germ cell tumor prevalence is 15-40% in patients with 45,X/46,XY mosaicism. We have previously reported bilateral gonadoblastoma in a 3 month old 45,X/46,XY child (PMID: 30646821). Dissecting gonadoblastoma of the ovary is characterized by a unique distribution of neoplastic germ cells. We report a child with bilateral dissecting gonadoblastoma. Clinical Case: This full-term neonate featured a 11x8 mm phallic structure, mildly rugated and fused labioscrotal folds, and a palpable gonad in the right inguinal canal. FISH analysis of 510 cells from a peripheral blood sample showed 507 cells with monosomy X and 3 cells with X and Y sex chromosomes. FISH studies on urine cells showed concordant results. Ultrasound revealed normal kidneys, an atypical appearing uterus, and bilateral oval hypoechoic to isoechoic structures lacking definite follicles in both adnexa. Shared decision-making discussions with parents concluded with decision for female sex of rearing. At 8 weeks of age, the phallus had lengthened and testosterone level was 120 ng/dl. Due to risk for germ cell tumors, bilateral gonadectomy was performed at 8 months of age. No further phallic growth was noted. Pathology showed left streak gonad with undifferentiated gonadal tissue/dissecting gonadoblastoma; right gonad with streak testis with undifferentiated gonadal tissue/dissecting gonadoblastoma. Fallopian tubes and associated Mullerian/Wolffian derivatives were identified. Cytogenetic analysis showed two cell lines: 45,X and another cell line with a structurally abnormal Y chromosome containing a duplication of the Yq11.23-qter segment. Staining showed numerous SALL4+ cells, some were also positive for OCT4, PLAP, C-Kitt and PgP9.5 stains. Conclusions: This infant’s phenotype and karyotype were troubling due to her risk for germ cell neoplasia. Based on our prior finding of bilateral gonadoblastoma in a 3-month infant, gonadectomy was performed at 8 months of age. Classical gonadoblastoma is considered to be non-invasive requiring only gonadectomy as treatment. Dissecting gonadoblastoma describes a pattern with either an infiltrative type or diffuse pattern instead of the typical small, nested arrangement. Dissecting gonadoblastoma can be confused with germinoma (PMID: 27454939). Correct identification of the histology is essential to avoid the more aggressive treatments often needed for germinoma. Monday, June 3, 2024