Such effects might be caused by the presence of salts or detergents in your samples (tissue extraction buffer). To test if this is the case you can run another sample (in another buffer) side by side on the same gel.

Thanks Viktor for your answer. 

The point is that this buffer (RIPA buffer) has been used by other people at the same time and they dind't have this problem. Anyway, I'll keep your recommedation in mind. 

Best wishes. 

Might be due to an excess of proteins in that area. In the low MW area, apart from your native proteins, you also have many fragments coming from the degradation of bigger proteins. In some staining techniques, such as ponceau or comassie, when you have a saturation in the system (too much protein) color, red in the case of Ponceau, does not appear and you get a big white spot instead of a red one, same that you have in your lower part of the gel. Try to run less amount of protein, maybe it would help.

Anecdotally, I may have seen a similar effect when my final concentration of SDS sample buffer (Laemmli sample buffer) has been too high or too low.

The final concentration in your sample when you load it should ideally be:

60 mM Tris-Cl pH 6.8, 2% SDS, 10% glycerol, 5% β-mercaptoethanol.

Here is a link for the recipe of 5x concentrated LB that we have been using:

http://www.cytographica.com/lab/solutions/5XSB.htm

Thanks David, I'll follow your advice. Maybe a little less of concrentration would work better.

It's interesting what you said Alex. I'm currently using the BioRad 2x Laemmli sample buffer and it's says that the final concentration it's:

65.8 mM Tris-HCl, pH 6.8, 2.1% SDS, 26.3% (w/v) glycerol, 0.01% bromophenol blue

They are considerably higher in some cases so I'll take in count. Thank you so much!