Determination of the molecular mass of the CSM identified by MoAb 103B2/9E10. Membrane preparations from a variety of hematopoietic and nonhematopoietic cell lines/preparations were separated by 10% SDS-polyacrylamide gel electrophoresis under nonreducing and reducing conditions and transferred to nitrocellulose. The filters were successively incubated with 103B2/9E10 MoAb supernatant and antimouse-HRPO, and the immunoreactive proteins were detected by ECL as described in Materials and Methods. Under nonreducing conditions (A), MoAb 103B2/9E10 identifies two differentially migrating species with an estimated molecular mass of 80 and 160 kD. However, upon reduction (B), the lower migrating 80-kD band is the major immunoreactive protein. (A and B) Lane 1, KG1a (progenitor cell line); lane 2, HL-60 (promyelocytic cell line); lane 3, Hel-DR (erythroleukemic); lane 4, K562 (erythroleukemic); lane 5, red blood cells; lane 6, Mo7e (megakaryocyte); lane 7, Jurkat (T-cell line); lane 8, CD19⁺ B cells; lane 9, HBMSCs; lane 10, human umbilical vein endothelial cells (HUVECs); lane 11, T1 (keratinocytes); lane 12, MG63 (osteosarcoma cell line).