It is difficult to obtain whole blood from mice no matter from tail vein or eye. I had collected whole blood from intestinal vein after abdominal exposure ten years ago, but the mice would die. Because you may obtain blood at different time points, I suggest using a microscope to help find tail vein.

you can view The Hydrodynamic Tail Vein Assay as a Tool for the Study of Liver Promoters and Enhancers

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I do collect CTCs from PDX mice blood at end point by collecting blood from the heart (terminal bleeding). This gives me around 1 ml of blood. I do RBCs lysis step followed by live immunostaining for the human marker CD298. Positive cells are then picked by a micromanipulator or a similar device.