Important Information Can Be Learned by Studying the Immune System

Important information can be learned by studying the immune system of female carriers of the XLA disease gene. These individuals, identified as mothers of boys with XLA, show non-random X chromosome inactivation in their B cells, but random X chromosome inactivation in all of their other cells, including their T cells and macrophages. This finding indicates that:

A) The BTK protein is expressed by bone marrow stromal cells that support B cell development.
B) The BTK protein is required in hematopoietic stem cells.
C) The BTK protein is required for mature B cells to enter lymphoid follicles.
D) B cells and T cells do not share a common progenitor.
E) B cells require the BTK protein for normal development.