Three groups (5 mice/group) of female BALB/C nude mice (age 4-6 weeks) were subcutaneously implanted with 1×107 of sh-CTL, sh-circPGR#1, sh-circPGR#2-infected MCF7 cells suspended in PBS. Each nude mouse was brushed with estrogen (E2, 10-2 M) every 3 days for the duration of the experiments to sustain xenograft tumor growth. All mice were euthanized 20 days after subcutaneous injection. Tumors were then excised, photographed and weighted. Animals were housed in the Animal Facility at Xiamen University under pathogen-free conditions, following the protocol approved by the Xiamen Animal Care and Use Committee.