The activation of components of the transcription factors such as AP-1 or c-jun is essential for a physiological response of metazoan cells during aging. The activity of such proto-oncoproteins is under enzymatic control. The function of c-jun is additionally modulated by the QM protein. Here, we studied the expression of the gene, encoding the QM-like protein in the sponge Suberites domuncula. These animals contain high levels of telomerase in their somatic cells. To understand the switch from telomerase-positive immortal cells to telomerase-negative mortal cells which undergo apoptosis, the expression of the QM-like gene was measured in this system. The cDNA, termed QMSD, encoding the QM-like protein was isolated from S. domuncula; its 642 bp long open reading frame encodes a putative protein, QM-SUBDO, of 24,702 Da. Phylogenetic analysis of the sponge QM-like protein revealed that it is closely related to other metazoan QM polypeptides and distinct from sequences of Eumycota or Viridiplantae. Our investigations demonstrated that in gemmules as well as in untreated tissue the expression of the QM-like gene is significantly higher than in tissue which undergoes induced apoptosis. The level of the QM-like protein even decreases drastically in cells that are induced to apoptosis (e.g. by cadmium). We suggest therefore that one event that is involved in the transition of sponge cells from their immortal telomerase-positive to the mortal telomerase-negative state may be the downregulation of the QM-like protein, a putative tumor suppressor polypeptide.