Because Bcl-2 family proteins function in the context of lipid membrane, it is important to examine their structure in the presence of membrane. No such structure is yet available due to technical difficulty. Toward this goal, Bcl-xL (Losonczi et al. 2000) and Bid (Oh et al. 2005) have been characterized under membranous conditions and, compared with the aqueous solution, exhibited significant structural differences.