Editor—In approximately 10% of patients with Silver-Russell syndrome,1-3 pre- and postnatal growth retardation with relative macrocephaly, triangular facies, and asymmetry is associated with maternal uniparental disomy of chromosome 7 (UPD(7)mat).4-7 The purpose of this report is to present a novel assay to diagnose UPD(7)mat by analysing the methylation status of PEG1/MEST, the only known imprinted gene on chromosome 7,8 which encodes a protein with sequence homology to alpha/ beta-hydrolase.9 10 Like its mouse homologuePeg1/Mest,10 11 the humanPEG1/MEST gene is expressed from the paternal allele but not from the maternal allele.12 The promoter of the paternal allele is unmethylated whereas that of the maternal allele is methylated.12 13 An exception to this strict correlation between the expression and methylation of the promoter is observed in lymphocytes, in which both the paternal and the maternal alleles of PEG1/MEST are expressed.12 14 Recently, we have shown that an alternative isoform of PEG1/MEST is expressed concurrently with the original isoform in lymphocytes and that the original isoform is expressed only from the paternal allele while the alternative isoform is expressed from both the paternal and maternal alleles.15 We concluded that parent of origin specific loss of the isoform 1 expression is strictly correlated with the methylation of the promoter of isoform 1. Documentation of this tight correlation validates the use of methylation analysis ofPEG1/MEST gene in …