Extended Data Figure 4. Additional examination of the Kit-MerCreMer knock-in allele and its potential leakiness in the absence of tamoxifen.

a, Histological analysis of eGFP fluorescent cells from the indicated tissues at day 28 from Kit^+/MCM × R-GFP mice that were given tamoxifen from 2 to 28 days. Nuclei are shown in blue and green shows eGFP fluorescing cells in the expected patterns for known regions of c-kit protein expression, such as the distinct pattern of melanocytes in the skin and widespread expression in the spleen and lungs. b, Immunohistochemistry in the testis of Kit^+/MCM × R-GFP mice for endogenous c-kit expression (red) versus cells that underwent recombination when tamoxifen was given by intraperitoneal injection (2 mg) for 5 consecutive days (green). The data show that most of the currently c-kit protein expressing cells in testis (only Leydig cells react, red surface staining) are also eGFP⁺ (intracellular), indicating that recombination only occurs in c-kit expressing cells, and the majority of them. c, Histological sections through the heart showing that the Kit-MerCreMer allele does not leak at baseline or after MI injury (n=3 mice per treatment). Kit^+/MCM × R-GFP mice were placed on tamoxifen-laden food or vehicle food beginning at 4 weeks of age and then subjected to MI injury 4 weeks later, followed by harvesting 4 weeks after that. In the presence of tamoxifen histological sections through the MI border zone of the heart show wide-spread eGFP⁺ cells (green) from the c-kit lineage (left panel), while in the absence of tamoxifen no eGFP⁺ cells are observed (right panel), hence the Kit-MerCreMer allele does not leak.