The present invention relates to the use of the epitope which comprises the tyrosine at position 1214 in the amino acid sequence of the vascular endothelial growth factor receptor, KDR/Flk-1, as a marker in the measurement of a change in the activation state of the KDR/Flk-1 receptor and to probes, such as antibodies, which recognize said epitope. The invention also relates to the use of KDR/Flk-1 epitope Y1214 as a marker in the detection of and/or measurement of the level of the KDR/Flk-1 receptor and to assays which utilize the use of the Y1214 epitope and to compounds derived from said assays.