With the increasing skepticism against transgenic technology and growing transgenic crops, TILLinG, based on chemically induced mutagenesis, has become the method of choice for detailed gene functional characterization and mutation breeding for crop improvement as it yields a range of alleles with different phenotypic strengths. The identification of the optimal dose of a chemical mutagen that maximizes the mutation frequency with acceptable plant viability is a key factor for the establishment of a good TILLinG population. To set up the cucumber TILLinG platform, we first performed a preliminary ‘‘kill curve’’ analysis and observed a strong correlation between the EMS dose and the seed germination rate. To maximize the genome mutation load and the plant survival, two EMS doses,MG132 were used to develop a reference EMS mutant collection under controlled conditions. Mutagenesis efficiency was assessed by scoring the occurrence of chlorotic and albino phenotypes. The observed rate of 0.6% of chlorotic and albino phenotypes in the mutant collection is in a similar range of previously described mutant collections and confirms the quality of the mutagenesis.. To validate the cucumber mutant collection, we screened for mutations in five genes and identified 26 independent alleles. As reported in other TILLinG studies,MDV3100 the EMS mutational specificity shows a strong preference for G/C to A/T transitions, 70 to 99% of the induced mutations. In our cucumber mutant population, most induced mutations were as expected, G/ C to A/T transitions, with the exception of the three following mutations, G/C to T/A, T/A to A/T and T/A to C/G. The spectrum of observed nucleotide changes is similar to the mutation spectrum observed in rice or tomato. Based on the TILLinG screens, we estimated the overall mutation density to one mutation every 1147 kb with an average of 5 alleles per gene. This mutation frequency is two fold lower than the rate reported for the closest cucurbit Cucumis melo, for tomato and for sunflower and equivalent to the rate of one mutation per megabase reported for barley.