Cells in UICollectionView overlap

I am using ViewController, in which I add a UICollectionView as follows:

UICollectionViewFlowLayout *collectionViewFlowLayout = [[UICollectionViewFlowLayout alloc] init];
[collectionViewFlowLayout setScrollDirection:UICollectionViewScrollDirectionVertical];
[collectionViewFlowLayout setMinimumInteritemSpacing:2];
[collectionViewFlowLayout setMinimumLineSpacing:4];
[collectionViewFlowLayout setHeaderReferenceSize:CGSizeMake(320, 30)];

CGRect collectionViewFrame = self.view.bounds;
self.collectionView = [[UICollectionView alloc] initWithFrame:collectionViewFrame collectionViewLayout:collectionViewFlowLayout];
self.collectionView.autoresizingMask = UIViewAutoresizingFlexibleHeight;

self.collectionView.delegate = self;
self.collectionView.dataSource = self;

[self.collectionView registerClass:[MyCollectionViewCell class] forCellWithReuseIdentifier:myCellIdentifier];

[self.view addSubview:self.collectionView];

And this is how I initialize my custom cell object (I made sure that the frame for each cell fills the entire width of the screen, so that the cells will be displayed one after another vertically in the UICollectionView):

CGSize mySize = CGSizeMake(320, 158);
self.myTitle = [[UILabel alloc ] initWithFrame:CGRectMake(0, 0, mySize.width, mySize.height)];
[self.myTitle setTextColor:[UIColor whiteColor]];
[self.myTitle setBackgroundColor:[UIColor greenColor]];
[self.myTitle setTextAlignment:NSTextAlignmentLeft];
[self.myTitle setNumberOfLines:1];
[self.myTitle setText:@"My Title"];
[self addSubview:self.myTitle];

This is how I initialize the cell frame in initWithNibName:

CGSize mySize = CGSizeMake(320, 158);
self = [super initWithFrame:CGRectMake(0, 0, mySize.width, mySize.height)];

And here is my cellForItemAtIndexPath code:

- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath {
UICollectionViewCell *cell = (UICollectionViewCell *)[collectionView dequeueReusableCellWithReuseIdentifier:myCellIdentifier forIndexPath:indexPath];

    return cell;
}

Now I have four of these cells, but they overlap. What am I doing wrong? Thank!