In E16.5 mouse embryos clc is expressed in muscles. Cryostat sections (A, E-L) or vibratome sections (B-D) from E16.5 (A-J) or E14.5 (K, L) were hybridized to clc (A, C, E, G, I and K) or clf (D, F, H, J and L). The control clc sense probe gave rise to very faint staining (B). Transverse section through the forelimb (A, K and L), the hindlimb (E and F) and saggital sections through the tongue (C and D) showing expression of clc and clf in muscles. The identity of the clc-positive cells such as muscle fibers was confirmed by double staining and compared to clf-positive cells. In situ hybridization using Dig-labeled probes for clc and clf (cytoplasmic blue staining) was performed on sections through shoulder muscles (G, H) or vibrissae (I, J) from E16.5 MLCnlacZ mice, which express the nlacZ reporter gene under the control of a muscle-specific MLC promoter. Subsequently, the sections were processed for immunohistochemical detection of β-galactosidase (nuclear brown staining). Arrows indicate double-labeled cells. Transverse section through the muzzle (I and J) shows that vibrissae (arrowheads) are positive for clc and clf whereas only clf is detected in muscles (asterisks) surrounding vibrissae. c, cartilage; m, muscle. Scale bars are 200 μm in A, E and F, 25 μm in G and H and 100 μm in I-L.