Everyone has cellular mutations. The vast majority don't replicate, our bodies reject them. With MGUS, a deranged plasma cell took root in our bone marrow and replicates (hopefully very slowly). I've had MGUS for +8 years (no telling how long the mutated plasma cell has been illegally parked), am 70, and MGUS doesn't limit me. I still bike thousands of miles a year, volunteer, and have fun. Did something cause the deranged plasma cell? I'll never know. Here are some risk factors: https://pmc.ncbi.nlm.nih.gov/articles/PMC2947966/#:~:text=The%20median%20age%20of%20diagnosis,are%20younger%20than%2040%20years.