Single nucleotide polymorphisms(SNPs)are the most abundant sequence variations found in plant genomes and are widely used as molecular genetic markers in cultivar identification and genetic diversity studies.The objective of this study was to identify SNP markers useful for discrimination of citrus cultivars,since large numbers of expressed sequence tags(ESTs)of sweet orange are available from the National Center for Biotechnology Information(NCBI).We now have the opportunity to discover SNP markers suitable for determining the haplotypes with which to distinguish very closely related cultivars and to assess genetic diversity within or between related species of citrus.SNPs and small insertions/deletions(Indels)from ESTs of sweet orange and satsuma were identified by the in silico SNP discovery strategy.55 296 EST sequences of sweet orange and 2 575 of satsuma retrieved from the NCBI repository were mined for potential SNPs.Cleaved amplified polymorphic sequences(CAPS)and sequencing approaches were used to validate putative SNPs in a sample of 30 citrus accessions.A total of 3 348 putative SNPs were identified based on the abundance of sequences and haplotype cosegregation.Of these 3 348 SNPs,the transitions,transversions and Indels ratios were 47.9,36.1 and 16.0%,respectively.The SNPs occurred on average at a frequency of 1 per 164 bp in the coding region of citrus.14 SNPs were randomly selected and genotyped according to 30 citrus accessions including 23 accessions of sweet orange;11 SNPs displayed polymorphism with an average polymorphism information content(PIC)of 0.20 among 30 citrus accessions.The genetic diversity present in sweet orange was low,so the 14 SNP markers failed to discriminate different cultivars of sweet orange,but they did succeed in distinguishing accessions of inter-species of citrus.In this study,SNPs were mined from EST sequences of sweet orange and satsuma,which displayed potential capability as molecular markers to discriminate inter-species accessions of citrus.It is anticipated that these putative SNPs could be applied in citrus genetics research and breeding.