SMN1 and proximal spinal muscular atrophy: For example, death in mice occursat the early stages of development [101],and death in egg-laying organisms, e.g. in D.melanogaster, occurs later, when the SMN protein level inherited fromthe mother decreases to a critical point [102].As expected, Smn knockout in a specific tissue leads to the maldevelopmentof this tissue and loss of a larger portion of its cellular component[103-105].Additional copies of SMN2 are usually inserted into the genome oftransgenic mice with SMA.