2D gels; upper left is the bacterially synthesized marker protein, shown throughout the figures as an arrow. Upper right is a 2D gel of a control lysate from OCI/AML-5 cells. Middle left is 2D gel of a lysate from ATRA-treated cells, showing the acidic isoform. Middle right is a 2D gel of an aliquot of lysate from ATRA-treated cells, digested with λ-phosphatase. It may be compared with the 2D control gel directly above it in the figure. The 2D gel at the bottom of the figure was made from a mixture of the two lysates above it; it shows the marker, a spot at the position of the native protein, but derived from λ-phosphatase–digested lysate from ATRA-treated cells and an acidic isoform, derived from the ATRA-treated cells.