Although numerous markers have been helpful in isolating and enriching spermatogonial stem cells (SSCs), such as Thy-1 and GFRα-1, no specific marker for this cell type has been identified so far. A 400-bp regulatory region of the stimulated by retinoic acid gene 8 (Stra8) promoter was reported to direct gene expression into SSCs and we have recently generated a new transgenic mouse model harboring the enhanced green fluorescent protein (EGFP) downstream of this Stra8 promoter. In this study, a detailed analysis of the EGFP expression pattern in the testis was carried out, showing that the transgene was expressed in meiotic and postmeiotic germ cells and not in undifferentiated germ cells. These findings were supported by confocal microscopy and flow cytometric analyses, and do not agree with the previous report concerning the 400-bp Stra8 promoter activity. © 2009 Wiley-Liss, Inc.