The X-linked genes accounting for the determination of the ovarian function are still undefined. In the last few years, a large interest has been dedicated to the BMP15 gene. BMP15 gene encodes for a TGFβ-like growth factor of oocyte origin with a critical role in female fertility in mammals and several other species. This gene maps to a locus on the short arm of X chromosome were locates several traits of TS, including ovarian failure. Several missense variations have been identified and experimentally proven as loss-of-function in the BMP15 signaling in animal models and humans. In women, BMP15 mutations occur frequently in association with primary or secondary amenorrhea in the setting of POI. Here, we studied the first variant identified in the BMP15 mature peptide causing the aminoacidic change R329C. By performing western blot, flow-cytometry analysis and a BMP-responsive luciferase-reporter assay on a human granulosa cell line, we evidenced a significant decreased secretion of the mature protein and a time-dependent intracellular accumulation of the precursor in presence of this mutation. Accordingly, the luciferase-reporter assay showed an impaired biological activity of the mutant on granulosa cells, with co-transfection experiments of WT and mutant BMP15 consistent with haploinsufficiency as pathogenic mechanism (similarly to previously reported BMP15 variants in humans and sheep). A concomitant analysis of 40 TS patients by array-CGH and FISH, permitted also the identification of a duplication of the only BMP15 gene in one 45,X patient with spontaneous menarche. The analysis of the rest of the cohort also revealed the presence of a mosaicism level with the euploid cell line >10% (providing a double copy of BMP15 gene in significant fractions of cells) only in the other 5 TS patients with spontaneous menarche. All other 34 TS patients with primary amenorrhea had only one copy of BMP15 gene in their genome. In conclusion, inactivating mutations of BMP15 can predispose to POI with haploinsufficiency as the main mechanism contributing to the ovarian failure whereas a double dose of BMP15 would be sufficient to partially overcome the deficiency of other X-linked genes. An adequate BMP15 gene dosage would be required for the determination of an adequate ovarian reserve, thus further supporting BMP15 as the first X-linked ovary-determining gene.