Cell protein expression levels. The color key used throughout the paper to denote the expression levels of Notch and Delta in each cell. Four cells labeled a, b, c and d are identified in the sheet of cells, and their corresponding expression levels shown in the color key. Cell a is a primary cell. Cell b is a secondary cell. Cell c is a defect since it is contacting only one primary cell. Cell d is a defect since it is touching another primary cell.