Yes, typically they are! However, chemical and enzymatic modifications such as deamidation and sialylation result in an increase in the net negative charge on the mAbs and cause/lead to formation of acidic variants. C-terminal lysine cleavage results in the loss of net positive charge and leads to acidic variant formation. Another mechanism for generating acidic variants is the formation of various types of covalent adducts. Here is the link so you can read more about it http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3011216/#!po=8.75000