Why are my protein bands lower than expected?

1. There may be activation, shearing and other activities in the protein, resulting in the final detected band size is different from the predicted size, you can find some target protein research reports to see how the size detected by others is.  

2. You need to determine whether the protein you want to detect is an endogenous protein or an exogenous protein, this can be directly searched on NCBI, if it is an exogenous protein, you need to check whether the sequence you find is a full sequence.  

3. There is also a possibility that you have an operation error when extracting the protein that causes the protein to degrade, and this can also lead to the detected bands lower than the predicted size.