BMP4 and retinoblastoma: The reason for the distinct responses of RB and P19EC cells to RA, BMP-4 and combined treatment can be explained by the fact that cell lines with a completely different genetic, species and tissue-specific background were analyzed: on the one hand RB cells derived from primary human retinoblastomas with mutations or a complete loss of the RB1 gene and on the other hand embryonic carcinoma cells derived from a murine teratocarcinoma.