The cause of embryonal rhabdomyosarcoma is currently unknown, but some studies have suggested that it may be caused by a gene or chromosomal variant. Certain specific genes or chromosomes are known to be associated with the development of rhabdomyosarcoma. Some embryonal rhabdomyosarcomas have complete heterozygous deletions to chromosome 11. Some genetic disorders and congenital syndromes are associated with an increased risk of developing rhabdomyosarcoma. Rhabdomyosarcomas are currently classified by histologic type into embryonal rhabdomyosarcoma, vesicular rhabdomyosarcoma, and mesenchymal rhabdomyosarcoma. Embryonal rhabdomyosarcoma, which accounts for 57% of all rhabdomyosarcoma cases, consists of the grape cluster cell type and the spindle cell type, and often occurs in the head and neck, bladder, vagina, prostate, and testes, and is predominant in children with rhabdomyosarcoma under the age of 5 years. The embryonal type has the best prognosis among rhabdomyosarcomas.