Ncbi-seg (a.k.a. SEG) is a program for identifying and masking segments of low compositional complexity in amino acid sequences.

ncbi-seg divides sequences into contrasting segments of low-complexity and high-complexity. Low-complexity segments defined by the algorithm represent "simple sequences" or "compositionally-biased regions".

This program is inappropriate for masking nucleotide sequences and, in fact, may strip some nucleotide ambiguity codes from nt. sequences as they are being read.