Immature teratomas grow tissues that resemble those found in an embryo and are the malignant cousins of the very common mature cystic teratomas or dermoid cysts. They may occur in combination with other germ cell tumors and are then called "mixed germ cell tumors." Pure immature teratoma is extremely rare and represents approximately 1% of all ovarian cancers, but within the germ cell tumor group, it is the second most common malignancy. In women under 20 years of age, these tumors represent 10% to 20% of all ovarian malignancies. Approximately half of all immature teratomas can occur even earlier, between the ages of 10 to 20 years. They rarely occur in postmenopausal women, though.