containing breeding line Massachusetts #2, for this gene was found in 'an aberrant plant of the breeding line Massachusetts #2' . This plant was resistant to race 4, but susceptible to another race, which later appeared to be 2 . 3 . 4 . 11 (Kerr & Patrick, 1977) . The Cf - 11 line was crossed to other (unknown) lines, resulting in Ontario 7716, which was used in later experiments . Upon inoculation of different plant genotypes with the systemic virus PVX, expressing the avirulence gene avr4, Vantage, Moneymaker - Cf - 4 (MM Cf - 4 ), Ontario 7522 and Ontario 7716 exhibited necrosis, indicating that these lines all contain an active Cf - 4 gene . Southern analysis with genomic DNA of these lines digested with EcoRl, Bg/ll, Hindlll and EcoRV and probed with a Cf - 4 or a Cf - 9 probe indicated that the patterns of MM Cf - 4 , Ontario 7522 and Ontario 7715 were identical and different from control lines . In experiments using PCR primers derived from the DNA sequence of Cf - 9 no difference in PCR pattern between MM Cf - 4 , Ontario 7522 and Ontario 7716 was detected again . Later, specific PCR primers were designed, based on the sequence of Cf - 4 . When used at high annealing temperatures, no difference was found between these lines, also after digestion of the PCR products with Bg /ll, Eco RI, Hin cll, Hind lll and Eco RV, while they did differ from banding patterns of other lines used . The experiments mentioned in this paper suggest the presence of a functional Cf - 4 gene in the Cf - 8 and Cf - 11 lines . The line Cf - 11 seems to harbour another gene, Cf - 11 , for which a corresponding avirulence gene exists . Because the Cf - 4 gene was already present in Massachusetts #2 and Cf - 11 was found in a plant of the same