By cytological and electron microscopy analyses of the Mer3, Mer4 (Gln170→stop), and control testes at various stages of spermatogenesis, we show that Merlin mutations affect meiotic cytokinesis of spermatocytes, cyst polarization and nuclear shaping during spermatid elongation, and spermatid individualization. This evidence concerns the gene NF2 and cyst.