While both tests are taken from an endometrial biopsy, the tests differ completely in purpose. The ERA (Endometrial Receptivity Analysis) is a test that evaluates the expression of genes in an attempt to define the optimal embryo transfer window.  While both tests are taken from an endometrial biopsy, the tests differ completely in purpose. The ERA (Endometrial Receptivity Analysis) is a test that evaluates the expression of genes in an attempt to define the optimal embryo transfer window.  The following graph highlights the key differences between the tests.