Recent developments with the L5178Y TK heterozygote mutagen assay system.

The presently available mutagen test systems are no longer receiving critical acclaim by sole virtue of their positive response to ethyl methanesulfonate (EMS). Rather, they are being asked to yield answers to such questions as: how mutagenic is EMS in this system and, given this number, what does it imply relative to the genetic risk of EMS to man? Both of these questions address themselves to the relevance of the test results, the first in a quantitative manner, the second qualitatively, in that it involves evaluating species' and systems' differences. Because this extrapolation of data must be to a rather complex organism-man-we are immediately'"face&'withagenetic quandary. Or'ganisms -which display thei-r'genetic repertoires in the simplest, most rapid manner-and which, therefore, are easiest to use and most definitive in the answers they yield-are phylogenetic foreigners to many. Used alone, one risks banning penicillin on the basis of toxic effects on gram-positive bacteria, or, to cite an example more oriented to mutagenicity testing, one might be tempted to place 5-bromodeoxyuridine (BUdR) on the GRAS list as a result of testing in thymidine kinase-deficient yeast. Yet, if we choose to eliminate most of this