Sarah Michelle Gellar and Shannen Doherty's friendship is an enviable one. The two actresses and longtime friends prove that good girlfriends are there for each other through thick and thin. Currently, that means Gellar lending her support during Doherty's intense battle with breast cancer.Doherty first shared news of her cancer diagnosis in February 2015. Earlier this month, the 45-year-old gave an emotional on-screen interview to ET about why she has been so public with her battle against the disease. "I wanted to put it out there the way it felt the best for me to put it out there," she said. "And also, if I could help one person, then it makes me go, 'Oh, okay.' It's easier to live with having cancer if I know I'm helping at least one person."On Monday, Gellar posted a selfie with Doherty on Instagram, sharing a touching message of support.She wrote, "My #mondaymuse (and really every day #muse) is my brave friend @theshando. I've always said she is one of the most loyal friends I've ever known, and now while she is facing the toughest journey of her life, she is sharing it with friends she has never met in the hopes of helping other people who are going through the same struggle. That's a true friend #FuckCancer."