PIK3CD and Merkel cell skin cancer: Thus, together with the recently reported pre-pro/pro- B- cell differentiation of MCC, i.e. TdT and PAX-5, immunoglobulin (Ig) expression and Ig rearrangements [18, 19], the functional PI3K p110δ expression in MCC might add another tesserae in the discussion of the cellular origin of MCC.