<n0:ComplexDefinition xmlns:n0="http://ncicb.nci.nih.gov/xml/owl/EVS/ComplexProperties.xsd#"><n0:def-definition>A substance that prevents cells from making or using progesterone (a hormone that plays a role in the menstrual cycle and pregnancy). Antiprogestins may stop some cancer cells from growing and they are being studied in the treatment of breast cancer. An antiprogestin is a type of hormone antagonist.</n0:def-definition><n0:def-source>NCI-GLOSS</n0:def-source></n0:ComplexDefinition>
<n0:ComplexDefinition xmlns:n0="http://ncicb.nci.nih.gov/xml/owl/EVS/ComplexProperties.xsd#"><n0:def-definition>A substance that blocks the activity of a progestin.</n0:def-definition><n0:def-source>NCI</n0:def-source></n0:ComplexDefinition>