Impaired development of CD4(+) CD8(+) thymocytes by csk-(knock-in) into fyn locus

p59(fyn) is one of the Src-family kinases thought to play an important role in signaling through T cell receptor, However, Fyn deficiency has caused no overt defects in vivo on T cell development, nor has it caused any changes in the phosphorylation status of molecules such as ZAP-70 which have been proposed as p59(fyn) substrates, This could be explained as being due to compensation of Fyn deficiency by other Src-family kinases, Here, we have 'knocked-in' the csk gene, a negative regulator of Src-family kinases, into fyn locus to challenge the problem of redundant functions among Src-family kinases, The csk-'knock-in' mice displayed atrophy of the thymic cortex and impaired development of CD4(+) CD8(+) thymocytes, This was concomitant with decrease in tyrosine phosphorylation of ZAP-70 and p120(cbl).