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A regional distribution of cells that express Sox9 in undifferentiated B6.Ytir gonads was observed. Sox9-positive cells appeared in the middle region, whereas they were absent from the cranial and caudal regions. Furthermore, cross sections of genital ridges at 11.5 and 12.5 dpc showed Sox9-positive cells among the innermost cells and rarely among coelomic epithelial cells. This distribution was similar in all embryos at 11.5 and 12.5 dpc, suggesting that regional location of pre-Sertoli cells in early genital ridges does not preclude their fate to differentiate as ovotestes or ovaries. The observed distribution of Sox9-positive pre-Sertoli cells in B6.Ytir genital ridges is similar to that of Sry-positive cells in morphologically undifferentiated genital ridges of normal XY mice. Thus the pattern of Sry expression followed by the location of Sox9-positive cells and the organization of the seminiferous cords [33, together with current results] suggests that the presence of short-range extracellular cues acting along the genital ridge during normal testis differentiation may be disturbed in B6.Ytir gonads. Alternatively, a cell autonomous effect due to weak Sry expression causing weak Sox9 transcription may occur Thus normal entrance of cells from the mesonephros into the normal developing testes may be partially or totally prevented in B6.Ytir gonads.