There is a significant level of crosstalk and functional overlap between the Gli TFs. Gli2 has been shown to compensate for the loss of Gli1 in transgenic Gli1-/- mice which are phenotypically normal. However, loss of Gli3 leads to abnormal patterning and loss of Gli2 affects the development of ventral cell types, most significantly in the floor plate. Gli2 has been shown to compensate for Gli1 ventrally and Gli3 dorsally in transgenic mice. Gli2 null mice embryos develop neural tube defects which, can be rescued by overexpression of Gli1 (Jacob and Briscoe, 2003). Gli1 has been shown to induce the two GLI2 α/β isoforms. Transgenic double homozygous Gli1-/- and Gli2-/- knockout mice display serious central nervous system and lung defects have small lungs, undescended testes, and a hopping gait as well as an extra postaxial nubbin on the limbs. Gli2-/- and Gli3-/- double homozygous transgenic mice are not viable and do not survive beyond embryonic level. These studies suggest overlapping roles for Gli1 with Gli2 and Gli2 with Gli3 in embryonic development. Transgenic Gli1-/- and Gli2-/- mice have a similar phenotype to transgenic Gli1 gain of function mice. This phenotype includes failure to thrive, early death, and a distended gut although no tumors form in transgenic Gli1-/- and Gli2-/- mice. This could suggest that overexpression of human Gli1 in the mouse may have led to a dominant negative rather than a gain-of-function phenotype. Transgenic mice over-expressing the transcription factor Gli2 under the K5 promoter in cutaneous keratinocytes develop multiple skin tumours on the ears, tail, trunk and dorsal aspect of the paw, resembling those of basal cell carcinoma (BCC). Unlike Gli1 transgenic mice, Gli2 transgenic mice only developed BCC-like tumors. Transgenic mice with N-terminal deletion of Gli2, developed the benign trichoblastomas, cylindromas and hamartomas but rarely developed BCCs. Gli2 is expressed in the interfollicular epidermis and the outer root sheath of hair follicles in normal human skin. This is significant as Shh regulates hair follicle growth and morphogenesis.