CytoTrace? Orange CMTMR is a rhodamine-based, cell-permeant, vital fluorescent dye that is retained by cells for prolonged periods (>72 hours) and is passed to daughter cells. Cells loaded with long-lasting dyes, including CMTMR, are used to follow cell trafficking and cell-to-cell interactions in vivo.^1,2,3 They have also been used to estimate the growth of implanted tumor spheroids.⁴ CytoTrace? Orange CMTMR displays excitation/emission maxima of 541/565 nm, respectively.