## 1 Introduction  
The importance of cancer stem cells (CSCs) in the development and recurrence of cancer has gained increased attention in cancer research during the last decade [1, 2]. A tumor contains a mixture of phenotypically heterogeneous cancer cells. Only a few cells in a tumor belong to the CSC population, but it appears that for many tumors of different origin it is mainly the CSCs that can give rise to new tumors with similar phenotypic composition as the original tumor [3]. Since the CSCs are drug resistant and favor cancer metastasis there is an increased need for new drugs and therapeutic strategies that reduce the CSCs specifically [4]. In breast cancer, a high expression of the cell surface marker CD44 in combination with a low expression of the cell surface marker CD24 have been used to identify CSCs [2; 5; 6]. The cationic ionophore salinomycin was found to target CSCs, in a high throughput screening study aimed to find drugs that target this subpopulation in breast cancer [7]. Subsequently, salinomycin has been found to target CSCs in many other human cancer types including leukemia [8], gastric cancer [9], colorectal cancer [10], osteosarcoma [11], pancreatic cancer [12; 13], prostate cancer [14; 15], head and neck squamous cell carcinoma [16], and lung cancer [17]. Many different mechanisms of action have been ascribed to salinomycin, such as disruption of actin stress fibers [18], downregulation of mRNAs and proteins related to stemness [19], impaired mitochondrial function, induction of autophagy, decreased ATP levels and increased reactive oxygen species production [20; 21; 22; 23], sequestration of iron in lysosomes [24], and induction of reactive oxygen species [23], with outcomes like decreased cell viability, proliferation, and migration, as well as stimulation of mesenchymal-to-epithelial-transition [25; 26; 27; 28; 29], Salinomycin has also been shown to increase the expression of E-cadherin [30; 31], a calcium-dependent cell adhesion glycoprotein molecule [32] found on the surface of epithelial cells [33]. The expression level of E-cadherin was inversely correlated with the tumorigenicity of different cell lines, where a high expression of E-cadherin correlated to low tumorigenicity and vice versa [34]. However, for clinical outcomes the role and correlation of E-cadherin is unclear [35] and the level of expression has been found to vary between tumors of different classification [35; 36]. We have previously found evidence for the molecular initiating event that explains most down-stream effects observed after salinomycin treatment [37]. Salinomycin was shown to almost immediately after addition to the cell culture medium localize to the endoplasmic reticulum (ER) of breast cancer cells [37], leading to increased cytosolic Ca\({}^{2+}\) levels followed by ER stress. This effect was down-stream linked to inhibition of the Wnt signaling pathway, which has previously been reported as an effect of salinomycin treatment [38]. These findings are deduced from population-based studies, but to increase our understanding of CSCs as well as how they can be targeted, more studies of single cell sensitivity are required. Digital holography is a quantitative phase imaging technique, which can be used to generate large amounts of information about individual cells based on the phase shift of light [39]. The method is label free, thus eliminating possible chemical toxicity, and no phototoxicity has been found. Therefore, the technique allows for long time imaging and when images are acquired with high time resolution they can be used for longitudinal tracking of individual cells, which is a powerful tool to investigate how individual cells in a population react to treatment over time. It can also be used to detect and over time trace subpopulations that react differently than the bulk of cells to drugs, e.g., that are drug insensitive and may therefore be the cause of metastasis [40; 41].