RARB and carcinoma: Interestingly, although cAMP has been described to be able to synergize with the induction of RAR-β by RA in embrionary carcinoma cells [51], we cannot observe this synergism in Schwann cells, in which maximum activation by the retinoid is already achieved in the absence of cAMP activators which, if any, have a negative impact on RAR-β expression levels, in a similar manner to what has been described in neuroblastoma cells [24].