specialize in (something)
To have specific training, education, proficiency, or experience in one particular subject, topic, field, etc. My bachelor's degree was in English literature overall, but I specialized in Victorian literature for my master's degree. Our company specializes in building networking strategies for small businesses.
specialize in something
to limit oneself to or be specially trained to practice one particular thing. I specialize in tropical medicine. What do you specialize in?