The phenotypic similarity between testes from bam and nup98-96 mutant animals and cyst cell-sSpi/EGFR-animals suggests that a mechanism exists that prevents the entry of spermatogonia into the spermatocyte-stage prior to the completion of exactly four rounds of TA-divisions and that the EGF pathway plays a role in this mechanism. The gene discussed is NUP98; the disease is cyst.