Sort the Following Events to Reflect the Order in Which

Short Answer

Sort the following events to reflect the order in which they typically take place during the reprogramming of fibroblasts to induced pluripotent stem cells. Your answer would be a four-letter string composed of letters A to D only, e.g. ADBC.
(A) Sustained endogenous expression of Oct4
(B) Gain of embryonic marker protein
(C) Loss of fibroblast-specific marker protein
(D) OSKM overexpression