IGF1 and retinoblastoma: Second, multiple GC boxes are present, which in the ELN promotor serve as binding site for the Sp1 (Specificity Protein1) member of the Sp transcription factor family and, together with activated Rb (Retinoblastoma), promote the transcription of ELN in response to TGFβ1 (Transforming Growth Factor beta 1) and IGF1 (insulin-like growth factor 1).