Muc21 expression prevented homotypic cell-cell and cell-ECM interactions in a TR-dependent manner. A, mock-transfected cells (diamonds), cells transiently expressing Muc21 with 4 TR (triangles) or Muc21 with 84 TR (circles) were compared for homotypic aggregation. The cells were untreated (open symbols) or treated with sialidase (filled symbols) and sorted to collect Venus-positive populations. More than 90% Venus-positive cells were used. B, adhesion assays were performed with the same preparations of cells as in A. The six consecutive columns represent, from left to right, mock transfectants, sialidase-treated mock transfectants, cells transiently expressing Muc21 with 4 TR before and after sialidase treatment, and cells transiently expressing Muc21 with 84 TR before and after sialidase treatment. BSA, bovine serum albumin.