However, the molecular basis underlying mammalian autophagy had remained cryptic until quite recently because of the difficulty in addressing this problem through a biochemical ap proach. As has been the case in other fields of membrane trafficking, a breakthrough came from the application of yeast genetics; more than 15 genes essential for yeast autophagy, AUT and APG were identified from genetic screens in yeast (see chapters 6 and 7). Database searches revealed that for most of the APG znàA UT gene products there are related proteins in mam mals, suggesting that the molecular machinery of autophagy may be conserved through evolu tion. The homologues are good candidates for functional counterparts of the yeast proteins and recent investigation of some of them revealed that they are actually involved in mamma lian autophagy. The identification of the homologues has begun a great expansion of our knowl edge about the molecular mechanism of autophagy, in particular as it relates to topics that are specific to mammalian cells.