Fifteen pairs of primers were designed to detect single nucleotide polymorphisms of exon 1 to exon 9 of progesterone receptor (PGR) gene in Small Tail Han,Hu,Dorset and Texel sheep by PCR-SSCP.Only the products amplified by primer P9 displayed polymorphism.For primer P9,three genotypes (AA,AG and GG) were detected in Small Tail Han,Hu and Dorset sheep,and two genotypes (AA and AG) in Texel sheep.Sequencing revealed one mutation (217A→G) of PGR gene in the genotype GG in comparison to the genotype AA,and this mutation did not cause any amino acid changes.The ewes with genotype GG or AG had 0.97 (P<0.05) or 0.64 (P<0.05) lambs more than those with genotype AA,and the difference of the litter size between GG and AG genotypes was non-significant (P>0.05) in Small Tail Han sheep.These results preliminarily indicated that allele G of PGR gene is a potential DNA marker for improving litter size in sheep.