Analysis of Rb phosphorylation, and expression of Cyclins A and B and RAG proteins in distinct subsets of human pre-T cells. Cellular lysates from human thymocytes of the indicated phenotypes were isolated as described in Materials and Methods and analyzed by Western blotting for the expression of Rb, Cyclin A, Cyclin B, RAG1, and RAG2. Expression of β-tubulin was analyzed as an internal control. Molecular sizes are indicated on the left (kD).