The cycle at which fluorescence from amplification exceeds the background fluorescence has been referred to as threshold cycle (Ct), crossing point (Cp), and take-off point (TOF) by different instrument manufacturers. But the term is now standardized by the MIQE guidelines as the quantification cycle( Cq). A lower Cq correlates with higher target expression in a sample.
Access the MIQE guidelines at:
Bustin SA, Benes V, et al. (2009) The MIQE Guidelines: minimum information for publication of quantitative real-time PCR experiments. Clin Chem, 55(4): 611−622. Updates: Bustin
SA, Beaulieu JF, et al. (2010) MIQE précis: Practical implementation of minimum standard guidelines for fluorescence-based quantitative real-time PCR experiments. BMC Mol Biol.11: 74–78, and Bustin SA, Benes V, et al. (2011) Primer sequence disclosure: A clarification of the MIQE Guidelines. Clin Chem, 57:919−921.