Some patients have large 13q deletions that contain the RB1 gene.[10] Germline or de novo mosaic 13q deletions can involve the surrounding genes delineating a contiguous gene syndrome characterized by retinoblastoma, developmental anomalies, and facial dysmorphisms.[11] Mosaicism results from postzygotic mutation that occurs during early embryonic development and can lead to germline or somatic mosaicism, potentially causing a less severe and/or variable phenotype compared with the equivalent constitutive mutation.[12]. This evidence concerns the gene RB1 and retinoblastoma.