(A) Suppression of the type X collagen gene expression in primary chondrocytes by the expression of exogenous fos genes. Chicken chondrocytes from the upper sternum (lanes 1–4) as well as CEF (lane 5) were infected just after preparation with a subgroup B virus carrying no exogenous gene [DS3(B), lane 1], or carrying chicken c-fos [FC1(B), lane 2], mouse c-fos [FM4(B), lane 3], or v-fos from FBJ-MSV [FJ-2(B), lane 4]. Total RNAs (8 μg) were prepared, separated on agarose gels, and hybridized with DNA probes encoding chicken type collagen (II) and type X collagen (X). (B) Western blot analysis of Fos proteins in CEF infected with subgroup A viruses such as DS3 (empty vector, lane 1), FM4 (c-fos virus, lane 2), FJ2 (v-fos virus, lane 4), CD3 (lane 5), and IR2-CD3 (lane 6), or a subgroup B virus, FM4(B) (c-fos virus, lane 3). Five days after infection, cells were disrupted under denaturing conditions, and each protein sample (20 μg) was separated in 10% polyacrylamide gel and analyzed. Molecular masses of c-Fos, v-Fos, and CD3 were 60, 55, and 35 kDa, respectively.