sentences:  By investigating hypomorphic mutants (which can survive 2 days longer), it was found that placental defects were primarily lethal and that there were also defects in cardiac and hepatic development, but many other organs were normal
  
  Conditional mutations can be produced in mice which affect only specific cells or tissues at specific times, so that the mouse can develop to adulthood and the adult phenotype can be studied.In one case, MED1 was found to participate in controlling the timing of events of meiosis in male mice.Conditional mutants in keratinocytes show differences in skin wound healing.