Complex sentence: Almost half of children with retinoblastoma have a hereditary genetic defect associated with retinoblastoma, in other cases, it is caused by a congenital mutation in the chromosome 13 gene.
Split into simpler sentences: Almost half of children with retinoblastoma have a hereditary genetic defect associated with retinoblastoma. In other cases, it is caused by a congenital mutation in the chromosome 13 gene.