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Ever since the early days of tissue culture, one of the chief debates has centered on the question of the maintenance and expression of tissueand organ-specific functions by animal cells grown in vitro (cf. ref. 1-4). This debate on "differentiation versus dedifferentiation," often confused by lack of both adequate experimental data and adequate definitions, can now be considered as settled by recent demonstrations of the possibility, in at least a few cases, of obtaining cultures capable of (a) expressing, over prolonged periods of time, differentiated functions characteristic of the tissue of origin and (b) re-expressing them after growth under conditions interfering with their expression.5-s These experiments thus demonstrate that in vitro cultured cells can oscillate between a state of overt differentiation (i.e. active performance of a given function) and a state of covert maintenance of the potentiality to express the same function under appropriate conditions (called "modulation" by Paul Weiss). The mechanism underlying the two states is sought today in the type of phenomena of regulation of gene expression which has recently been uncovered in bacteria and with which they have many features in common.3 Research along these lines is in progress in this and several other laboratories (cf. ref. 10). The debate on "differentiation-dedifferentiation" in vitro has meanwhile ob-

proceedings of the national academy of sciences of the united states of america

RETENTION OF MULTIPLE DEVELOPMENTAL POTENTIALITIES BY CELLS OF A MOUSE TESTICULAR TERATOCARCINOMA DURING PROLONGED CULTURE <i>in vitro</i> AND THEIR EXTINCTION UPON HYBRIDIZATION WITH CELLS OF PERMANENT LINES

RETENTION OF MULTIPLE DEVELOPMENTAL POTENTIALITIES BY CELLS OF A MOUSE TESTICULAR TERATOCARCINOMA DURING PROLONGED CULTURE in vitro AND THEIR EXTINCTION UPON HYBRIDIZATION WITH CELLS OF PERMANENT LINES