We previously demonstrated maternal monoallelic expression of the Wilms tumor suppressor gene, WT1, in about half of pre-term placental villus and fetal brain tissues examined. There were two alternative explanations for this pattern of the WT1 expression, i.e., an imprinting polymorphism vs. a developmental stage-dependent switching from monoallelic to biallelic expression of the gene. To investigate these possibilities, we examined WT1 expression in a larger number of villus samples (46 samples) with gestational ages ranging from 4 to 21 weeks, using reverse transcriptase-based polymerase chain reaction (RT-PCR) to amplify the sequences for polymorphic sites in the 3'-untranslated region (UTR) of WT1. Maternal monoallelic expression was observed in 7 (39%) of 18 samples informative for the polymorphism, while the expression of the remaining 11 samples was biallelic. In addition, there was no correlation between expression patterns and gestational ages of the samples. The results indicate that the pattern of expression (monoallelic vs. biallelic) is polymorphic. The expression patterns were also studied in five different organs from a 21-week-old fetus, showing monoallelic expression only in the placenta and biallelic expression in other organs (heart, lung, liver and intestine). The finding supports the tissue specificity of the WT1 monoallelic expression.