Objective: The objective of the proposed study is to assess the effectiveness of a 4-month yoga program compared to a waiting list control group on musculoskeletal complaints in women with hormone-positive stage I-III breast cancer receiving endocrine treatment who report musculoskeletal complaints. Study design: The COBRA study is a randomized controlled trial with two study arms: a yoga- and a waiting list control group. Study population: For this study, 140 women with oestrogen-receptor positive stage I-III breast cancer on endocrine treatment (aromatase inhibitors; \>4 months) will be recruited. These women experience musculoskeletal complaints (\>3 months) which started or exacerbated after initiation of endocrine treatment. Furthermore, eligible women did not practice yoga in the last six months and are not planning to start yoga and are not highly physically active (i.e., \>150 minutes per week moderate-vigorous exercise). Intervention: The intervention consists of two one-hour sessions/week supervised yoga and once a week 30-minute yoga exercises at home. The 4-month yoga program will be an active form of yoga, such as Easy Vinyasa yoga, which is characterized by continuous slow movements linked with breathing. The waiting list control patients will be offered an online yoga program after the 4-month study period.