Hello, there are many genes involved in the regulation of muscle development. The balance between hypertrophy and atrophy is due (in the adult stage) to IGF-1, which leads to a protein synthesis in muscle (Hypertrophy) and myostatin, which inhibits protein synthesis (atrophy). Deregulation of these genes can lead to an atrophy phenotype (more myostatin and less IGF-1). Myostatin inhibits myogenic factors (MRF): MyoD, Myf4, myf5 and myogenin, which are pro-hypertrophic. An inhibition of these MRFs therefore causes atrophy.