Objective       To construct a myostatin (MSTN) knockout mice

determine the effect of the gene knockout on the skeletal muscle growth in lactating mice

and investigate the relative mechanism. Methods      The Cre-loxp system was used to generate Mstn knockout mice (Mstn-/- mice)

and then the genotype of the offspring was identified by PCR. The weight of body and leg skeletal muscles of Mstn-/- mice were compared with those of the wild-type mice. The sizes of leg skeletal muscle fibers were tested in the Mstn-/- and wild-type mice by immunofluorescence assay. The mRNA expression levels of Atrogin-1 and MuRF-1 were detected by qPCR. Results      The results of PCR showed that the genotype of the offspring of Mstn-/- mice was in accordance with MSTN-loxP+/+ MCK-Cre+/-. Compared with the wild-type mice

the body weight Mstn-/- mice were significantly heavier on the 7th and 21st days (P0.05)

and the weight and sizes of leg skeletal muscle fibers were also bigger (P0.01). Besides

the mRNA levels of Atrogin-1 and MuRF-1 of were significantly decreased after knocking out of Mstn (P0.01). Conclusion      A Mstn-knockout mice model is successfully constructed. Mstn-/- mice have thickening skeletal muscle fibers

higher weight of leg skeletal muscle and significantly increased body weight

which may be mediated by down-regulation of ubiquitin ligase Atrogin-1 and MuRF-1.