Such games have been utilized as a learning and development tool for medical purposes. The first game in the series was considered by several journalists to be one of the most influential video games of the first decade of the 21st century. The series has sold more than 25 million units worldwide, earning US$2 billion at retail, claimed by Activision in 2009 to be the 3rd largest game franchise after the Mario and Madden NFL franchises; the third main title of the series, Guitar Hero III: Legends of Rock, is also claimed by Activision to be the first single video game title to exceed $1 billion in sales.