创建一个名为MyDog的类,使用方法setBreed,该方法将字符串作为参数,并设置名为breed的类变量的值.
Here is the code for the MyDog class in Python:

```python
class MyDog:
    def __init__(self):
        self.breed = None
        
    def setBreed(self, breed):
        self.breed = breed
```

To use this class, you can create a new instance of the MyDog class, then call the "setBreed" method with a string argument:

```python
my_dog = MyDog()
my_dog.setBreed("Labrador")
```