Single spermatocytes and small clusters of spermatocytes were detected in 95% of the cyst cell-sSpi/EGFR-testes (n = 76) but were not found in testes from w1118-animals (n = 80), in testes from cyst cell-sSpi/EGFR-animals prior to the temperature shift (n = 37), or in control testes from animals carrying only the Gal4-transactivators (n = 42) or only the UAS-constructs (n = 58) shifted to 29°C. The gene discussed is EGFR; the disease is cyst.